diff --git a/.editorconfig b/.editorconfig
index c58ac87..ee3e4f5 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -110,7 +110,7 @@ csharp_style_prefer_null_check_over_type_check = true : suggestion
 csharp_style_implicit_object_creation_when_type_is_apparent = true : suggestion
 
 # primary constructors (C# 12+)
-csharp_style_prefer_primary_constructors = true : suggestion
+csharp_style_prefer_primary_constructors = true : warning
 
 # collection expressions (C# 12+)
 dotnet_style_prefer_collection_expression = true : suggestion
diff --git a/src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryServiceTests.cs b/src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryServiceTests.cs
index c2a1374..c455dc3 100644
--- a/src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryServiceTests.cs
+++ b/src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryServiceTests.cs
@@ -115,7 +115,7 @@ public void RecordDeliveryAttempt_UpdatesDeliveryStatus()
         var attempt = new DeliveryAttempt
         {
             AttemptNumber = 1,
-            AttemptTime = DateTime.UtcNow,
+            AttemptTime = DateTimeOffset.UtcNow,
             Outcome = DeliveryOutcome.Success,
             HttpStatusCode = 200,
         };
@@ -136,7 +136,7 @@ public void RecordDeliveryAttempt_NonExistingEvent_DoesNotThrow()
         var attempt = new DeliveryAttempt
         {
             AttemptNumber = 1,
-            AttemptTime = DateTime.UtcNow,
+            AttemptTime = DateTimeOffset.UtcNow,
             Outcome = DeliveryOutcome.Success,
         };
 
@@ -283,7 +283,7 @@ private static SimulatorEvent CreateTestEvent(string id)
                 Id = id,
                 Subject = "/test/subject",
                 EventType = "Test.EventType",
-                EventTime = DateTime.UtcNow.ToString("o"),
+                EventTime = DateTimeOffset.UtcNow.ToString("o"),
                 DataVersion = "1.0",
                 Data = new { Property = "Value" },
             }
diff --git a/src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryStoreTests.cs b/src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryStoreTests.cs
index c67bb51..9a2825c 100644
--- a/src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryStoreTests.cs
+++ b/src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryStoreTests.cs
@@ -258,7 +258,7 @@ private static EventHistoryRecord CreateTestRecord(string id, string topicName =
             EventType = "Test.EventType",
             Subject = "/test/subject",
             Source = "/test/source",
-            EventTime = DateTime.UtcNow.ToString("o"),
+            EventTime = DateTimeOffset.UtcNow.ToString("o"),
             InputSchema = EventSchema.EventGridSchema,
             PayloadJson = "{}",
         };
diff --git a/src/AzureEventGridSimulator.Tests/Helpers/FakeTimeProvider.cs b/src/AzureEventGridSimulator.Tests/Helpers/FakeTimeProvider.cs
new file mode 100644
index 0000000..67f4cbe
--- /dev/null
+++ b/src/AzureEventGridSimulator.Tests/Helpers/FakeTimeProvider.cs
@@ -0,0 +1,31 @@
+namespace AzureEventGridSimulator.Tests.Helpers;
+
+/// <summary>
+/// A fake TimeProvider for testing time-dependent code.
+/// Allows tests to control the current time for deterministic behavior.
+/// </summary>
+public class FakeTimeProvider(DateTimeOffset startTime) : TimeProvider
+{
+    private DateTimeOffset _utcNow = startTime;
+
+    public override DateTimeOffset GetUtcNow()
+    {
+        return _utcNow;
+    }
+
+    /// <summary>
+    /// Sets the current time to a specific value.
+    /// </summary>
+    public void SetUtcNow(DateTimeOffset value)
+    {
+        _utcNow = value;
+    }
+
+    /// <summary>
+    /// Advances the current time by the specified duration.
+    /// </summary>
+    public void Advance(TimeSpan duration)
+    {
+        _utcNow = _utcNow.Add(duration);
+    }
+}
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/CloudEventSchemaFormatterTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/CloudEventSchemaFormatterTests.cs
index fa2efbd..5fa3407 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/CloudEventSchemaFormatterTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/CloudEventSchemaFormatterTests.cs
@@ -72,8 +72,8 @@ public void GivenEventGridEvent_WhenSerializedAsCloudEvent_ThenConvertedCorrectl
             EventType = "Test.Event.Type",
             EventTime = "2025-01-15T10:30:00Z",
             Data = new { Property = "Value" },
-            Topic = "/test/topic",
         };
+        eventGridEvent.SetTopic("/test/topic");
 
         var simulatorEvent = SimulatorEvent.FromEventGridEvent(eventGridEvent);
         var json = _formatter.Serialize(simulatorEvent);
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/SimulatorEventTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/SimulatorEventTests.cs
index f75ef96..ac6824e 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/SimulatorEventTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/SimulatorEventTests.cs
@@ -18,8 +18,8 @@ public void GivenEventGridEvent_WhenWrappedInSimulatorEvent_ThenPropertiesMapped
             EventTime = "2025-01-15T10:30:00Z",
             Data = new { Property = "Value" },
             DataVersion = "1.0",
-            Topic = "/test/topic",
         };
+        eventGridEvent.SetTopic("/test/topic");
 
         var simulatorEvent = SimulatorEvent.FromEventGridEvent(eventGridEvent);
 
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Commands/SendNotificationEventsToSubscriberCommandHandlerTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Commands/SendNotificationEventsToSubscriberCommandHandlerTests.cs
index 5294e4e..a7d3708 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Commands/SendNotificationEventsToSubscriberCommandHandlerTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Commands/SendNotificationEventsToSubscriberCommandHandlerTests.cs
@@ -407,7 +407,7 @@ private static SimulatorEvent CreateTestEventGridEvent(string id)
                 Id = id,
                 Subject = "/test/subject",
                 EventType = "Test.EventType",
-                EventTime = DateTime.UtcNow.ToString("o"),
+                EventTime = DateTimeOffset.UtcNow.ToString("o"),
                 DataVersion = "1.0",
                 Data = new { Property = "Value" },
             }
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Commands/ValidateSubscriptionCommandHandlerTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Commands/ValidateSubscriptionCommandHandlerTests.cs
index 94cd777..7e4ba7e 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Commands/ValidateSubscriptionCommandHandlerTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Commands/ValidateSubscriptionCommandHandlerTests.cs
@@ -16,7 +16,7 @@ public class ValidateSubscriptionCommandHandlerTests
     public ValidateSubscriptionCommandHandlerTests()
     {
         _logger = Substitute.For<ILogger<ValidateSubscriptionCommandHandler>>();
-        _handler = new ValidateSubscriptionCommandHandler(_logger);
+        _handler = new ValidateSubscriptionCommandHandler(TimeProvider.System, _logger);
     }
 
     [Fact]
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridSchemaFormatterTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridSchemaFormatterTests.cs
index 5fced06..ea3dad5 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridSchemaFormatterTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridSchemaFormatterTests.cs
@@ -9,7 +9,7 @@ namespace AzureEventGridSimulator.Tests.UnitTests.EventGrid;
 [Trait("Category", "unit")]
 public class EventGridSchemaFormatterTests
 {
-    private readonly EventGridSchemaFormatter _formatter = new();
+    private readonly EventGridSchemaFormatter _formatter = new(TimeProvider.System);
 
     [Fact]
     public void GivenEventGridEvent_WhenSerialized_ThenJsonContainsAllFields()
@@ -22,9 +22,9 @@ public void GivenEventGridEvent_WhenSerialized_ThenJsonContainsAllFields()
             EventTime = "2025-01-15T10:30:00Z",
             Data = new { Property = "Value" },
             DataVersion = "1.0",
-            Topic = "/test/topic",
             MetadataVersion = "1",
         };
+        eventGridEvent.SetTopic("/test/topic");
 
         var simulatorEvent = SimulatorEvent.FromEventGridEvent(eventGridEvent);
         var json = _formatter.Serialize(simulatorEvent);
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridValidationTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridValidationTests.cs
index 44acc3d..0958219 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridValidationTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridValidationTests.cs
@@ -1,3 +1,4 @@
+using System.Text.Json;
 using AzureEventGridSimulator.Domain.Entities;
 using Shouldly;
 using Xunit;
@@ -258,36 +259,55 @@ string metadataVersion
     public void GivenEventWithNonNullTopic_WhenValidated_ThenExceptionThrown()
     {
         // Topic should be null/empty when publishing - it's set by the service
-        var eventGridEvent = new EventGridEvent
-        {
-            Id = "test-id-123",
-            Subject = "/test/subject",
-            EventType = "Test.EventType",
-            EventTime = "2025-01-15T10:30:00Z",
-            Topic = "/subscriptions/some/topic",
-        };
-
-        var exception = Should.Throw<InvalidOperationException>(() => eventGridEvent.Validate());
+        // Use JSON deserialization to simulate a publisher sending a Topic value
+        var json = """
+            {
+                "id": "test-id-123",
+                "subject": "/test/subject",
+                "eventType": "Test.EventType",
+                "eventTime": "2025-01-15T10:30:00Z",
+                "topic": "/subscriptions/some/topic"
+            }
+            """;
+        var eventGridEvent = JsonSerializer.Deserialize<EventGridEvent>(json);
+
+        var exception = Should.Throw<InvalidOperationException>(() => eventGridEvent!.Validate());
         exception.Message.ShouldContain("Topic");
     }
 
-    [Theory]
-    [InlineData(null)]
-    [InlineData("")]
-    public void GivenEventWithNullOrEmptyTopic_WhenValidated_ThenNoExceptionThrown(string topic)
+    [Fact]
+    public void GivenEventWithNullTopic_WhenValidated_ThenNoExceptionThrown()
     {
+        // Topic defaults to null when not set
         var eventGridEvent = new EventGridEvent
         {
             Id = "test-id-123",
             Subject = "/test/subject",
             EventType = "Test.EventType",
             EventTime = "2025-01-15T10:30:00Z",
-            Topic = topic,
         };
 
         Should.NotThrow(() => eventGridEvent.Validate());
     }
 
+    [Fact]
+    public void GivenEventWithEmptyTopic_WhenValidated_ThenNoExceptionThrown()
+    {
+        // Use JSON deserialization to set empty Topic
+        var json = """
+            {
+                "id": "test-id-123",
+                "subject": "/test/subject",
+                "eventType": "Test.EventType",
+                "eventTime": "2025-01-15T10:30:00Z",
+                "topic": ""
+            }
+            """;
+        var eventGridEvent = JsonSerializer.Deserialize<EventGridEvent>(json);
+
+        Should.NotThrow(() => eventGridEvent!.Validate());
+    }
+
     [Fact]
     public void GivenEventWithNullData_WhenValidated_ThenNoExceptionThrown()
     {
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Filtering/AdvancedFilterEventAcceptanceTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Filtering/AdvancedFilterEventAcceptanceTests.cs
index 80c1af3..a31a2a4 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Filtering/AdvancedFilterEventAcceptanceTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Filtering/AdvancedFilterEventAcceptanceTests.cs
@@ -9,25 +9,31 @@ namespace AzureEventGridSimulator.Tests.UnitTests.Filtering;
 [Trait("Category", "unit")]
 public class AdvancedFilterEventAcceptanceTests
 {
-    private static readonly EventGridEvent _gridEvent = new()
+    private static readonly EventGridEvent _gridEvent = CreateTestEvent();
+
+    private static EventGridEvent CreateTestEvent()
     {
-        Id = "EventId",
-        Data = new
+        var evt = new EventGridEvent
         {
-            NumberValue = 1,
-            IsTrue = true,
-            Name = "StringValue",
-            DoubleValue = 0.12345d,
-            NumberMaxValue = ulong.MaxValue,
-            SubObject = new { Id = 1, Name = "Test" },
-        },
-        DataVersion = "5.0",
-        EventTime = DateTime.UtcNow.ToString("O"),
-        EventType = "this.is.a.test.event.type",
-        MetadataVersion = "2.3.4",
-        Subject = "TheEventSubject",
-        Topic = "THE_EVENT_TOPIC",
-    };
+            Id = "EventId",
+            Data = new
+            {
+                NumberValue = 1,
+                IsTrue = true,
+                Name = "StringValue",
+                DoubleValue = 0.12345d,
+                NumberMaxValue = ulong.MaxValue,
+                SubObject = new { Id = 1, Name = "Test" },
+            },
+            DataVersion = "5.0",
+            EventTime = DateTimeOffset.UtcNow.ToString("O"),
+            EventType = "this.is.a.test.event.type",
+            MetadataVersion = "2.3.4",
+            Subject = "TheEventSubject",
+        };
+        evt.SetTopic("THE_EVENT_TOPIC");
+        return evt;
+    }
 
     [Theory]
     [ClassData(typeof(PositiveFilterTestCaseContainer))]
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAegSasTokenTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAegSasTokenTests.cs
index ddd3ee5..76c9a9b 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAegSasTokenTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAegSasTokenTests.cs
@@ -14,7 +14,7 @@ public void GivenValidAegSasToken_WhenValidated_ThenReturnsTrue()
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
@@ -29,7 +29,7 @@ public void GivenExpiredAegSasToken_WhenValidated_ThenReturnsFalse()
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(-5)
+            DateTimeOffset.UtcNow.AddMinutes(-5)
         );
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
@@ -44,7 +44,7 @@ public void GivenAegSasTokenWithWrongSignature_WhenValidated_ThenReturnsFalse()
         var token = GenerateValidSasToken(
             "WrongKey123456789012345=",
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
@@ -59,7 +59,7 @@ public void GivenInvalidAegSasToken_WhenValidated_ThenLogsError()
         var token = GenerateValidSasToken(
             "WrongKey123456789012345=",
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
@@ -82,7 +82,7 @@ public void GivenAegSasTokenWithDifferentCase_WhenValidated_ThenHeaderIsFoundCas
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary { { "AEG-SAS-TOKEN", token } };
 
@@ -94,7 +94,7 @@ public void GivenAegSasTokenWithDifferentCase_WhenValidated_ThenHeaderIsFoundCas
     [Fact]
     public void GivenTokenExpiringExactlyNow_WhenValidated_ThenReturnsFalse()
     {
-        var token = GenerateValidSasToken(ValidTopicKey, "http://localhost", DateTime.UtcNow);
+        var token = GenerateValidSasToken(ValidTopicKey, "http://localhost", DateTimeOffset.UtcNow);
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
         var result = Validator.IsValid(headers, ValidTopicKey);
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAuthorizationHeaderTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAuthorizationHeaderTests.cs
index 2bdfc12..eb25f5f 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAuthorizationHeaderTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAuthorizationHeaderTests.cs
@@ -15,7 +15,7 @@ public void GivenValidAuthorizationHeader_WhenValidated_ThenReturnsTrue()
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary
         {
@@ -33,7 +33,7 @@ public void GivenExpiredAuthorizationHeader_WhenValidated_ThenReturnsFalse()
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(-5)
+            DateTimeOffset.UtcNow.AddMinutes(-5)
         );
         var headers = new HeaderDictionary
         {
@@ -51,7 +51,7 @@ public void GivenAuthorizationHeaderWithWrongSignature_WhenValidated_ThenReturns
         var token = GenerateValidSasToken(
             "WrongKey123456789012345=",
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary
         {
@@ -69,7 +69,7 @@ public void GivenInvalidAuthorizationHeader_WhenValidated_ThenLogsError()
         var token = GenerateValidSasToken(
             "WrongKey123456789012345=",
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary
         {
@@ -108,7 +108,7 @@ public void GivenAuthorizationHeaderWithDifferentCase_WhenValidated_ThenHeaderIs
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary
         {
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorTestBase.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorTestBase.cs
index 8525cd8..91c9650 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorTestBase.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorTestBase.cs
@@ -15,12 +15,16 @@ public abstract class SasKeyValidatorTestBase
     protected SasKeyValidatorTestBase()
     {
         Logger = Substitute.For<ILogger<SasKeyValidator>>();
-        Validator = new SasKeyValidator(Logger);
+        Validator = new SasKeyValidator(TimeProvider.System, Logger);
     }
 
-    protected static string GenerateValidSasToken(string key, string resource, DateTime expiry)
+    protected static string GenerateValidSasToken(
+        string key,
+        string resource,
+        DateTimeOffset expiry
+    )
     {
-        var decodedExpiration = expiry.ToString("o");
+        var decodedExpiration = expiry.UtcDateTime.ToString("o");
 
         var encodedResource = HttpUtility.UrlEncode(resource);
         var encodedExpiration = HttpUtility.UrlEncode(decodedExpiration);
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/DeadLetterServiceTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/DeadLetterServiceTests.cs
index afa57bf..2a3a294 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/DeadLetterServiceTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/DeadLetterServiceTests.cs
@@ -68,7 +68,7 @@ private static PendingDelivery CreatePendingDelivery(
                 Id = eventId ?? Guid.NewGuid().ToString(),
                 Subject = "test/subject",
                 EventType = "Test.EventType",
-                EventTime = DateTime.UtcNow.ToString("o"),
+                EventTime = DateTimeOffset.UtcNow.ToString("o"),
                 DataVersion = "1.0",
                 Data = new { test = "data" },
             }
@@ -130,7 +130,7 @@ public async Task GivenDelivery_WhenWriting_ThenFileContainsCorrectJson()
             new DeliveryAttempt
             {
                 AttemptNumber = 5,
-                AttemptTime = DateTime.UtcNow,
+                AttemptTime = DateTimeOffset.UtcNow,
                 Outcome = DeliveryOutcome.HttpError,
                 HttpStatusCode = 503,
                 ErrorMessage = "Service Unavailable",
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/HttpEventDeliveryServiceTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/HttpEventDeliveryServiceTests.cs
index 2752140..8eea3fd 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/HttpEventDeliveryServiceTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/HttpEventDeliveryServiceTests.cs
@@ -23,7 +23,7 @@ public HttpEventDeliveryServiceTests()
     {
         _logger = Substitute.For<ILogger<HttpEventDeliveryService>>();
         _formatterFactory = new EventSchemaFormatterFactory(
-            new EventGridSchemaFormatter(),
+            new EventGridSchemaFormatter(TimeProvider.System),
             new CloudEventSchemaFormatter()
         );
     }
@@ -264,7 +264,7 @@ private static SimulatorEvent CreateTestEvent()
                 Id = Guid.NewGuid().ToString(),
                 Subject = "test/subject",
                 EventType = "Test.EventType",
-                EventTime = DateTime.UtcNow.ToString("o"),
+                EventTime = DateTimeOffset.UtcNow.ToString("o"),
                 DataVersion = "1.0",
                 Data = new { test = "data" },
             }
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/InMemoryDeliveryQueueTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/InMemoryDeliveryQueueTests.cs
index 3750ccc..ea31629 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/InMemoryDeliveryQueueTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/InMemoryDeliveryQueueTests.cs
@@ -2,6 +2,7 @@
 using AzureEventGridSimulator.Domain.Services.Retry;
 using AzureEventGridSimulator.Infrastructure.Settings;
 using AzureEventGridSimulator.Infrastructure.Settings.Subscribers;
+using AzureEventGridSimulator.Tests.Helpers;
 using NSubstitute;
 using Shouldly;
 using Xunit;
@@ -11,16 +12,18 @@ namespace AzureEventGridSimulator.Tests.UnitTests.Retry;
 [Trait("Category", "unit")]
 public class InMemoryDeliveryQueueTests
 {
+    private static readonly DateTimeOffset FixedTime = new(2025, 1, 15, 12, 0, 0, TimeSpan.Zero);
     private readonly ILogger<InMemoryDeliveryQueue> _logger;
     private readonly InMemoryDeliveryQueue _queue;
+    private readonly FakeTimeProvider _timeProvider = new(FixedTime);
 
     public InMemoryDeliveryQueueTests()
     {
         _logger = Substitute.For<ILogger<InMemoryDeliveryQueue>>();
-        _queue = new InMemoryDeliveryQueue(_logger);
+        _queue = new InMemoryDeliveryQueue(_timeProvider, _logger);
     }
 
-    private static PendingDelivery CreatePendingDelivery()
+    private static PendingDelivery CreatePendingDelivery(DateTimeOffset? nextAttemptTime = null)
     {
         var subscriber = new HttpSubscriberSettings
         {
@@ -43,7 +46,7 @@ private static PendingDelivery CreatePendingDelivery()
                 Id = Guid.NewGuid().ToString(),
                 Subject = "test/subject",
                 EventType = "Test.EventType",
-                EventTime = DateTime.UtcNow.ToString("o"),
+                EventTime = FixedTime.ToString("o"),
                 DataVersion = "1.0",
                 Data = new { test = "data" },
             }
@@ -55,6 +58,8 @@ private static PendingDelivery CreatePendingDelivery()
             Subscriber = subscriber,
             Topic = topic,
             InputSchema = EventSchema.EventGridSchema,
+            EnqueuedTime = FixedTime,
+            NextAttemptTime = nextAttemptTime ?? FixedTime,
         };
     }
 
@@ -131,7 +136,7 @@ public void GivenExistingDelivery_WhenRequeuing_ThenUpdatesDelivery()
         var delivery = CreatePendingDelivery();
         _queue.Enqueue(delivery);
 
-        var newNextAttemptTime = DateTime.UtcNow.AddMinutes(5);
+        var newNextAttemptTime = FixedTime.AddMinutes(5);
         delivery.NextAttemptTime = newNextAttemptTime;
         delivery.AttemptCount = 2;
 
@@ -163,8 +168,7 @@ public void GivenNoDeliveries_WhenGettingDueDeliveries_ThenReturnsEmpty()
     [Fact]
     public void GivenDeliveryDueNow_WhenGettingDueDeliveries_ThenReturnsDelivery()
     {
-        var delivery = CreatePendingDelivery();
-        delivery.NextAttemptTime = DateTime.UtcNow.AddSeconds(-1); // Due 1 second ago
+        var delivery = CreatePendingDelivery(FixedTime.AddSeconds(-1));
         _queue.Enqueue(delivery);
 
         var dueDeliveries = _queue.GetDueDeliveries().ToList();
@@ -176,8 +180,7 @@ public void GivenDeliveryDueNow_WhenGettingDueDeliveries_ThenReturnsDelivery()
     [Fact]
     public void GivenDeliveryNotYetDue_WhenGettingDueDeliveries_ThenReturnsEmpty()
     {
-        var delivery = CreatePendingDelivery();
-        delivery.NextAttemptTime = DateTime.UtcNow.AddMinutes(5); // Due in 5 minutes
+        var delivery = CreatePendingDelivery(FixedTime.AddMinutes(5));
         _queue.Enqueue(delivery);
 
         var dueDeliveries = _queue.GetDueDeliveries().ToList();
@@ -188,14 +191,9 @@ public void GivenDeliveryNotYetDue_WhenGettingDueDeliveries_ThenReturnsEmpty()
     [Fact]
     public void GivenMixedDueAndNotDue_WhenGettingDueDeliveries_ThenReturnsOnlyDue()
     {
-        var dueDelivery1 = CreatePendingDelivery();
-        dueDelivery1.NextAttemptTime = DateTime.UtcNow.AddSeconds(-10);
-
-        var dueDelivery2 = CreatePendingDelivery();
-        dueDelivery2.NextAttemptTime = DateTime.UtcNow.AddSeconds(-5);
-
-        var notDueDelivery = CreatePendingDelivery();
-        notDueDelivery.NextAttemptTime = DateTime.UtcNow.AddMinutes(5);
+        var dueDelivery1 = CreatePendingDelivery(FixedTime.AddSeconds(-10));
+        var dueDelivery2 = CreatePendingDelivery(FixedTime.AddSeconds(-5));
+        var notDueDelivery = CreatePendingDelivery(FixedTime.AddMinutes(5));
 
         _queue.Enqueue(dueDelivery1);
         _queue.Enqueue(dueDelivery2);
@@ -212,14 +210,9 @@ public void GivenMixedDueAndNotDue_WhenGettingDueDeliveries_ThenReturnsOnlyDue()
     [Fact]
     public void GivenMultipleDueDeliveries_WhenGettingDueDeliveries_ThenOrderedByNextAttemptTime()
     {
-        var delivery1 = CreatePendingDelivery();
-        delivery1.NextAttemptTime = DateTime.UtcNow.AddSeconds(-5);
-
-        var delivery2 = CreatePendingDelivery();
-        delivery2.NextAttemptTime = DateTime.UtcNow.AddSeconds(-10); // Earlier
-
-        var delivery3 = CreatePendingDelivery();
-        delivery3.NextAttemptTime = DateTime.UtcNow.AddSeconds(-1); // Latest
+        var delivery1 = CreatePendingDelivery(FixedTime.AddSeconds(-5));
+        var delivery2 = CreatePendingDelivery(FixedTime.AddSeconds(-10)); // Earliest
+        var delivery3 = CreatePendingDelivery(FixedTime.AddSeconds(-1)); // Latest
 
         _queue.Enqueue(delivery1);
         _queue.Enqueue(delivery2);
@@ -232,4 +225,22 @@ public void GivenMultipleDueDeliveries_WhenGettingDueDeliveries_ThenOrderedByNex
         dueDeliveries[1].Id.ShouldBe(delivery1.Id);
         dueDeliveries[2].Id.ShouldBe(delivery3.Id); // Latest last
     }
+
+    [Fact]
+    public void GivenDeliveryInFuture_WhenTimeAdvances_ThenBecomesDue()
+    {
+        var delivery = CreatePendingDelivery(FixedTime.AddMinutes(5));
+        _queue.Enqueue(delivery);
+
+        // Initially not due
+        _queue.GetDueDeliveries().ShouldBeEmpty();
+
+        // Advance time by 6 minutes
+        _timeProvider.Advance(TimeSpan.FromMinutes(6));
+
+        // Now should be due
+        var dueDeliveries = _queue.GetDueDeliveries().ToList();
+        dueDeliveries.Count.ShouldBe(1);
+        dueDeliveries[0].Id.ShouldBe(delivery.Id);
+    }
 }
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/PendingDeliveryTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/PendingDeliveryTests.cs
index aada4bc..f3d83bd 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/PendingDeliveryTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/PendingDeliveryTests.cs
@@ -9,11 +9,13 @@ namespace AzureEventGridSimulator.Tests.UnitTests.Retry;
 [Trait("Category", "unit")]
 public class PendingDeliveryTests
 {
+    private static readonly DateTimeOffset FixedTime = new(2025, 1, 15, 12, 0, 0, TimeSpan.Zero);
+
     private static PendingDelivery CreatePendingDelivery(
         int? ttlMinutes = 1440,
         int? maxAttempts = 30,
         bool? retryEnabled = true,
-        DateTime? enqueuedTime = null
+        DateTimeOffset? enqueuedTime = null
     )
     {
         RetryPolicySettings retryPolicy = null;
@@ -50,59 +52,46 @@ private static PendingDelivery CreatePendingDelivery(
                 Id = Guid.NewGuid().ToString(),
                 Subject = "test/subject",
                 EventType = "Test.EventType",
-                EventTime = DateTime.UtcNow.ToString("o"),
+                EventTime = FixedTime.ToString("o"),
                 DataVersion = "1.0",
                 Data = new { test = "data" },
             }
         );
 
-        var delivery = new PendingDelivery
+        return new PendingDelivery
         {
             Event = evt,
             Subscriber = subscriber,
             Topic = topic,
             InputSchema = EventSchema.EventGridSchema,
+            EnqueuedTime = enqueuedTime ?? FixedTime,
+            NextAttemptTime = enqueuedTime ?? FixedTime,
         };
-
-        // Set EnqueuedTime if provided (using object initializer since it's init-only)
-        if (enqueuedTime.HasValue)
-        {
-            return new PendingDelivery
-            {
-                Event = evt,
-                Subscriber = subscriber,
-                Topic = topic,
-                InputSchema = EventSchema.EventGridSchema,
-                EnqueuedTime = enqueuedTime.Value,
-            };
-        }
-
-        return delivery;
     }
 
     [Fact]
     public void GivenNewDelivery_WhenChecking_ThenIsNotExpired()
     {
-        var delivery = CreatePendingDelivery(60);
+        var delivery = CreatePendingDelivery(60, enqueuedTime: FixedTime);
 
-        delivery.IsExpired.ShouldBeFalse();
+        delivery.IsExpired(FixedTime).ShouldBeFalse();
     }
 
     [Fact]
     public void GivenDeliveryOlderThanTtl_WhenChecking_ThenIsExpired()
     {
-        var delivery = CreatePendingDelivery(60, enqueuedTime: DateTime.UtcNow.AddMinutes(-61));
+        var delivery = CreatePendingDelivery(60, enqueuedTime: FixedTime.AddMinutes(-61));
 
-        delivery.IsExpired.ShouldBeTrue();
+        delivery.IsExpired(FixedTime).ShouldBeTrue();
     }
 
     [Fact]
     public void GivenDeliveryJustBeforeTtl_WhenChecking_ThenIsNotExpired()
     {
-        var delivery = CreatePendingDelivery(60, enqueuedTime: DateTime.UtcNow.AddMinutes(-59));
+        var delivery = CreatePendingDelivery(60, enqueuedTime: FixedTime.AddMinutes(-59));
 
         // Just before TTL, should not be expired yet
-        delivery.IsExpired.ShouldBeFalse();
+        delivery.IsExpired(FixedTime).ShouldBeFalse();
     }
 
     [Fact]
@@ -111,10 +100,10 @@ public void GivenDeliveryWithDefaultTtl_WhenChecking_ThenExpiredAfter24Hours()
         // Default TTL is 1440 minutes (24 hours)
         var delivery = CreatePendingDelivery(
             null, // Use default
-            enqueuedTime: DateTime.UtcNow.AddMinutes(-1441)
+            enqueuedTime: FixedTime.AddMinutes(-1441)
         );
 
-        delivery.IsExpired.ShouldBeTrue();
+        delivery.IsExpired(FixedTime).ShouldBeTrue();
     }
 
     [Fact]
@@ -122,10 +111,10 @@ public void GivenDeliveryWithShortTtl_WhenChecking_ThenExpiredQuickly()
     {
         var delivery = CreatePendingDelivery(
             1, // 1 minute TTL
-            enqueuedTime: DateTime.UtcNow.AddMinutes(-2)
+            enqueuedTime: FixedTime.AddMinutes(-2)
         );
 
-        delivery.IsExpired.ShouldBeTrue();
+        delivery.IsExpired(FixedTime).ShouldBeTrue();
     }
 
     [Fact]
@@ -225,7 +214,7 @@ public void GivenOneAttempt_WhenGettingLastAttempt_ThenReturnsThatAttempt()
         var attempt = new DeliveryAttempt
         {
             AttemptNumber = 1,
-            AttemptTime = DateTime.UtcNow,
+            AttemptTime = FixedTime,
             Outcome = DeliveryOutcome.HttpError,
             HttpStatusCode = 500,
         };
@@ -241,20 +230,20 @@ public void GivenMultipleAttempts_WhenGettingLastAttempt_ThenReturnsLast()
         var attempt1 = new DeliveryAttempt
         {
             AttemptNumber = 1,
-            AttemptTime = DateTime.UtcNow.AddMinutes(-10),
+            AttemptTime = FixedTime.AddMinutes(-10),
             Outcome = DeliveryOutcome.HttpError,
             HttpStatusCode = 500,
         };
         var attempt2 = new DeliveryAttempt
         {
             AttemptNumber = 2,
-            AttemptTime = DateTime.UtcNow.AddMinutes(-5),
+            AttemptTime = FixedTime.AddMinutes(-5),
             Outcome = DeliveryOutcome.Timeout,
         };
         var attempt3 = new DeliveryAttempt
         {
             AttemptNumber = 3,
-            AttemptTime = DateTime.UtcNow,
+            AttemptTime = FixedTime,
             Outcome = DeliveryOutcome.HttpError,
             HttpStatusCode = 503,
         };
@@ -283,13 +272,7 @@ public void GivenNewDelivery_WhenCreated_ThenHasCorrectDefaults()
 
         delivery.AttemptCount.ShouldBe(0);
         delivery.Attempts.ShouldBeEmpty();
-        delivery.EnqueuedTime.ShouldBeInRange(
-            DateTime.UtcNow.AddSeconds(-1),
-            DateTime.UtcNow.AddSeconds(1)
-        );
-        delivery.NextAttemptTime.ShouldBeInRange(
-            DateTime.UtcNow.AddSeconds(-1),
-            DateTime.UtcNow.AddSeconds(1)
-        );
+        delivery.EnqueuedTime.ShouldBe(FixedTime);
+        delivery.NextAttemptTime.ShouldBe(FixedTime);
     }
 }
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/RetrySchedulerTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/RetrySchedulerTests.cs
index 742f2f3..5a454e6 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Retry/RetrySchedulerTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Retry/RetrySchedulerTests.cs
@@ -1,4 +1,5 @@
 using AzureEventGridSimulator.Domain.Services.Retry;
+using AzureEventGridSimulator.Tests.Helpers;
 using Shouldly;
 using Xunit;
 
@@ -7,6 +8,15 @@ namespace AzureEventGridSimulator.Tests.UnitTests.Retry;
 [Trait("Category", "unit")]
 public class RetrySchedulerTests
 {
+    private static readonly DateTimeOffset FixedTime = new(2025, 1, 15, 12, 0, 0, TimeSpan.Zero);
+    private readonly RetryScheduler _scheduler;
+    private readonly FakeTimeProvider _timeProvider = new(FixedTime);
+
+    public RetrySchedulerTests()
+    {
+        _scheduler = new RetryScheduler(_timeProvider);
+    }
+
     [Theory]
     [InlineData(1, 10)] // 10 seconds
     [InlineData(2, 30)] // 30 seconds
@@ -23,16 +33,10 @@ public void GivenAttemptNumber_WhenGettingNextRetryTime_ThenFollowsAzureSchedule
         int expectedDelaySeconds
     )
     {
-        var before = DateTime.UtcNow;
+        var nextRetryTime = _scheduler.GetNextRetryTime(attemptNumber);
 
-        var nextRetryTime = RetryScheduler.GetNextRetryTime(attemptNumber);
-
-        var after = DateTime.UtcNow;
-        var expectedMin = before.AddSeconds(expectedDelaySeconds);
-        var expectedMax = after.AddSeconds(expectedDelaySeconds + 1); // Allow 1 second tolerance
-
-        nextRetryTime.ShouldBeGreaterThanOrEqualTo(expectedMin);
-        nextRetryTime.ShouldBeLessThanOrEqualTo(expectedMax);
+        var expectedTime = FixedTime.AddSeconds(expectedDelaySeconds);
+        nextRetryTime.ShouldBe(expectedTime);
     }
 
     [Theory]
@@ -44,17 +48,12 @@ public void GivenAttemptBeyondSchedule_WhenGettingNextRetryTime_ThenReturns12Hou
         int attemptNumber
     )
     {
-        var before = DateTime.UtcNow;
         var expectedDelaySeconds = 12 * 60 * 60; // 12 hours
 
-        var nextRetryTime = RetryScheduler.GetNextRetryTime(attemptNumber);
+        var nextRetryTime = _scheduler.GetNextRetryTime(attemptNumber);
 
-        var after = DateTime.UtcNow;
-        var expectedMin = before.AddSeconds(expectedDelaySeconds);
-        var expectedMax = after.AddSeconds(expectedDelaySeconds + 1);
-
-        nextRetryTime.ShouldBeGreaterThanOrEqualTo(expectedMin);
-        nextRetryTime.ShouldBeLessThanOrEqualTo(expectedMax);
+        var expectedTime = FixedTime.AddSeconds(expectedDelaySeconds);
+        nextRetryTime.ShouldBe(expectedTime);
     }
 
     [Theory]
@@ -64,59 +63,57 @@ public void GivenInvalidAttemptNumber_WhenGettingNextRetryTime_ThenReturnsImmedi
         int attemptNumber
     )
     {
-        var before = DateTime.UtcNow;
-
-        var nextRetryTime = RetryScheduler.GetNextRetryTime(attemptNumber);
+        var nextRetryTime = _scheduler.GetNextRetryTime(attemptNumber);
 
-        nextRetryTime.ShouldBeLessThanOrEqualTo(before.AddSeconds(1));
+        nextRetryTime.ShouldBe(FixedTime);
     }
 
     [Fact]
     public void GivenHttp404_WhenGettingNextRetryTime_ThenMinimum5MinuteDelay()
     {
-        var before = DateTime.UtcNow;
         var expectedMinDelaySeconds = 5 * 60; // 5 minutes
 
         // First attempt would normally be 10s, but 404 requires minimum 5 minutes
-        var nextRetryTime = RetryScheduler.GetNextRetryTime(1, 404);
+        var nextRetryTime = _scheduler.GetNextRetryTime(1, 404);
 
-        nextRetryTime.ShouldBeGreaterThanOrEqualTo(before.AddSeconds(expectedMinDelaySeconds));
+        var expectedTime = FixedTime.AddSeconds(expectedMinDelaySeconds);
+        nextRetryTime.ShouldBe(expectedTime);
     }
 
     [Fact]
     public void GivenHttp408_WhenGettingNextRetryTime_ThenMinimum2MinuteDelay()
     {
-        var before = DateTime.UtcNow;
         var expectedMinDelaySeconds = 2 * 60; // 2 minutes
 
         // First attempt would normally be 10s, but 408 requires minimum 2 minutes
-        var nextRetryTime = RetryScheduler.GetNextRetryTime(1, 408);
+        var nextRetryTime = _scheduler.GetNextRetryTime(1, 408);
 
-        nextRetryTime.ShouldBeGreaterThanOrEqualTo(before.AddSeconds(expectedMinDelaySeconds));
+        var expectedTime = FixedTime.AddSeconds(expectedMinDelaySeconds);
+        nextRetryTime.ShouldBe(expectedTime);
     }
 
     [Fact]
     public void GivenHttp503_WhenGettingNextRetryTime_ThenMinimum30SecondDelay()
     {
-        var before = DateTime.UtcNow;
         var expectedMinDelaySeconds = 30; // 30 seconds
 
         // First attempt would normally be 10s, but 503 requires minimum 30 seconds
-        var nextRetryTime = RetryScheduler.GetNextRetryTime(1, 503);
+        var nextRetryTime = _scheduler.GetNextRetryTime(1, 503);
 
-        nextRetryTime.ShouldBeGreaterThanOrEqualTo(before.AddSeconds(expectedMinDelaySeconds));
+        var expectedTime = FixedTime.AddSeconds(expectedMinDelaySeconds);
+        nextRetryTime.ShouldBe(expectedTime);
     }
 
     [Fact]
     public void GivenHttp503OnLaterAttempt_WhenStandardDelayIsLarger_ThenUsesStandardDelay()
     {
-        var before = DateTime.UtcNow;
         var expectedMinDelaySeconds = 5 * 60; // 5 minutes (attempt 4 standard delay)
 
         // Attempt 4 standard delay is 5 minutes, which is larger than 503's 30 seconds
-        var nextRetryTime = RetryScheduler.GetNextRetryTime(4, 503);
+        var nextRetryTime = _scheduler.GetNextRetryTime(4, 503);
 
-        nextRetryTime.ShouldBeGreaterThanOrEqualTo(before.AddSeconds(expectedMinDelaySeconds));
+        var expectedTime = FixedTime.AddSeconds(expectedMinDelaySeconds);
+        nextRetryTime.ShouldBe(expectedTime);
     }
 
     [Theory]
@@ -127,7 +124,7 @@ public void GivenHttp503OnLaterAttempt_WhenStandardDelayIsLarger_ThenUsesStandar
     [InlineData(204)]
     public void GivenSuccessStatusCode_WhenChecking_ThenReturnsTrue(int statusCode)
     {
-        RetryScheduler.IsSuccessStatusCode(statusCode).ShouldBeTrue();
+        _scheduler.IsSuccessStatusCode(statusCode).ShouldBeTrue();
     }
 
     [Theory]
@@ -140,7 +137,7 @@ public void GivenSuccessStatusCode_WhenChecking_ThenReturnsTrue(int statusCode)
     [InlineData(503)]
     public void GivenNonSuccessStatusCode_WhenChecking_ThenReturnsFalse(int statusCode)
     {
-        RetryScheduler.IsSuccessStatusCode(statusCode).ShouldBeFalse();
+        _scheduler.IsSuccessStatusCode(statusCode).ShouldBeFalse();
     }
 
     [Theory]
@@ -150,7 +147,7 @@ public void GivenNonSuccessStatusCode_WhenChecking_ThenReturnsFalse(int statusCo
     [InlineData(413)]
     public void GivenImmediateDeadLetterStatusCode_WhenChecking_ThenReturnsTrue(int statusCode)
     {
-        RetryScheduler.ShouldImmediatelyDeadLetter(statusCode).ShouldBeTrue();
+        _scheduler.ShouldImmediatelyDeadLetter(statusCode).ShouldBeTrue();
     }
 
     [Theory]
@@ -163,7 +160,7 @@ public void GivenImmediateDeadLetterStatusCode_WhenChecking_ThenReturnsTrue(int
     [InlineData(504)]
     public void GivenRetryableStatusCode_WhenChecking_ThenReturnsFalse(int statusCode)
     {
-        RetryScheduler.ShouldImmediatelyDeadLetter(statusCode).ShouldBeFalse();
+        _scheduler.ShouldImmediatelyDeadLetter(statusCode).ShouldBeFalse();
     }
 
     [Theory]
@@ -176,7 +173,7 @@ public void GivenKnownStatusCode_WhenGettingDeadLetterReason_ThenReturnsSpecific
         string expectedReason
     )
     {
-        RetryScheduler.GetDeadLetterReasonForStatusCode(statusCode).ShouldBe(expectedReason);
+        _scheduler.GetDeadLetterReasonForStatusCode(statusCode).ShouldBe(expectedReason);
     }
 
     [Theory]
@@ -188,6 +185,6 @@ public void GivenUnknownStatusCode_WhenGettingDeadLetterReason_ThenReturnsGeneri
         string expectedReason
     )
     {
-        RetryScheduler.GetDeadLetterReasonForStatusCode(statusCode).ShouldBe(expectedReason);
+        _scheduler.GetDeadLetterReasonForStatusCode(statusCode).ShouldBe(expectedReason);
     }
 }
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/EventHubEventDeliveryServiceTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/EventHubEventDeliveryServiceTests.cs
index 9521512..4dbefe3 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/EventHubEventDeliveryServiceTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/EventHubEventDeliveryServiceTests.cs
@@ -21,7 +21,7 @@ public EventHubEventDeliveryServiceTests()
     {
         _logger = Substitute.For<ILogger<EventHubEventDeliveryService>>();
         _formatterFactory = new EventSchemaFormatterFactory(
-            new EventGridSchemaFormatter(),
+            new EventGridSchemaFormatter(TimeProvider.System),
             new CloudEventSchemaFormatter()
         );
         _propertyResolver = new DeliveryPropertyResolver();
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/ServiceBusEventDeliveryServiceTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/ServiceBusEventDeliveryServiceTests.cs
index 5c3beeb..e4c8335 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/ServiceBusEventDeliveryServiceTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/ServiceBusEventDeliveryServiceTests.cs
@@ -21,7 +21,7 @@ public ServiceBusEventDeliveryServiceTests()
     {
         _logger = Substitute.For<ILogger<ServiceBusEventDeliveryService>>();
         _formatterFactory = new EventSchemaFormatterFactory(
-            new EventGridSchemaFormatter(),
+            new EventGridSchemaFormatter(TimeProvider.System),
             new CloudEventSchemaFormatter()
         );
         _propertyResolver = new DeliveryPropertyResolver();
diff --git a/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/StorageQueueEventDeliveryServiceTests.cs b/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/StorageQueueEventDeliveryServiceTests.cs
index 6f81deb..52350ff 100644
--- a/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/StorageQueueEventDeliveryServiceTests.cs
+++ b/src/AzureEventGridSimulator.Tests/UnitTests/Subscribers/Delivery/StorageQueueEventDeliveryServiceTests.cs
@@ -21,7 +21,7 @@ public StorageQueueEventDeliveryServiceTests()
     {
         _logger = Substitute.For<ILogger<StorageQueueEventDeliveryService>>();
         _formatterFactory = new EventSchemaFormatterFactory(
-            new EventGridSchemaFormatter(),
+            new EventGridSchemaFormatter(TimeProvider.System),
             new CloudEventSchemaFormatter()
         );
         _service = new StorageQueueEventDeliveryService(_logger, _formatterFactory);
diff --git a/src/AzureEventGridSimulator/Domain/Commands/SendNotificationEventsToSubscriberCommandHandler.cs b/src/AzureEventGridSimulator/Domain/Commands/SendNotificationEventsToSubscriberCommandHandler.cs
index a6c3442..bd3a8dc 100644
--- a/src/AzureEventGridSimulator/Domain/Commands/SendNotificationEventsToSubscriberCommandHandler.cs
+++ b/src/AzureEventGridSimulator/Domain/Commands/SendNotificationEventsToSubscriberCommandHandler.cs
@@ -164,7 +164,7 @@ private static void EnrichEvents(SimulatorEvent[] events, string topicName)
         {
             if (evt.Schema == EventSchema.EventGridSchema && evt.EventGridEvent != null)
             {
-                evt.EventGridEvent.Topic = topicPath;
+                evt.EventGridEvent.SetTopic(topicPath);
                 evt.EventGridEvent.MetadataVersion = "1";
             }
             else if (evt.Schema == EventSchema.CloudEventV1_0 && evt.CloudEvent != null)
diff --git a/src/AzureEventGridSimulator/Domain/Commands/ValidateAllSubscriptionsCommandHandler.cs b/src/AzureEventGridSimulator/Domain/Commands/ValidateAllSubscriptionsCommandHandler.cs
index 5d411ff..7ae0a7b 100644
--- a/src/AzureEventGridSimulator/Domain/Commands/ValidateAllSubscriptionsCommandHandler.cs
+++ b/src/AzureEventGridSimulator/Domain/Commands/ValidateAllSubscriptionsCommandHandler.cs
@@ -16,7 +16,8 @@ public class ValidateAllSubscriptionsCommandHandler(
     ILogger<ValidateAllSubscriptionsCommandHandler> logger,
     IHttpClientFactory httpClientFactory,
     SimulatorSettings simulatorSettings,
-    ValidationIpAddressProvider validationIpAddress
+    ValidationIpAddressProvider validationIpAddress,
+    TimeProvider timeProvider
 ) : IRequestHandler<ValidateAllSubscriptionsCommand>
 {
     public async Task Handle(
@@ -55,7 +56,7 @@ HttpSubscriberSettings subscription
 
             var evt = new EventGridEvent
             {
-                EventTime = DateTime.UtcNow.ToString("O"),
+                EventTime = timeProvider.GetUtcNow().ToString("o"),
                 DataVersion = "1",
                 EventType = "Microsoft.EventGrid.SubscriptionValidationEvent",
                 Id = Guid.NewGuid().ToString(),
diff --git a/src/AzureEventGridSimulator/Domain/Commands/ValidateSubscriptionCommandHandler.cs b/src/AzureEventGridSimulator/Domain/Commands/ValidateSubscriptionCommandHandler.cs
index 7356e84..2cc8889 100644
--- a/src/AzureEventGridSimulator/Domain/Commands/ValidateSubscriptionCommandHandler.cs
+++ b/src/AzureEventGridSimulator/Domain/Commands/ValidateSubscriptionCommandHandler.cs
@@ -3,8 +3,10 @@
 
 namespace AzureEventGridSimulator.Domain.Commands;
 
-public class ValidateSubscriptionCommandHandler(ILogger<ValidateSubscriptionCommandHandler> logger)
-    : IRequestHandler<ValidateSubscriptionCommand, bool>
+public class ValidateSubscriptionCommandHandler(
+    TimeProvider timeProvider,
+    ILogger<ValidateSubscriptionCommandHandler> logger
+) : IRequestHandler<ValidateSubscriptionCommand, bool>
 {
     public Task<bool> Handle(
         ValidateSubscriptionCommand request,
@@ -19,7 +21,7 @@ CancellationToken cancellationToken
         if (
             subscriber != null
             && subscriber.ValidationCode == request.ValidationCode
-            && !subscriber.ValidationPeriodExpired
+            && !subscriber.ValidationPeriodExpired(timeProvider.GetUtcNow())
         )
         {
             subscriber.ValidationStatus = SubscriptionValidationStatus.ValidationSuccessful;
diff --git a/src/AzureEventGridSimulator/Domain/Entities/CloudEvent.cs b/src/AzureEventGridSimulator/Domain/Entities/CloudEvent.cs
index 382a756..af18a15 100644
--- a/src/AzureEventGridSimulator/Domain/Entities/CloudEvent.cs
+++ b/src/AzureEventGridSimulator/Domain/Entities/CloudEvent.cs
@@ -1,3 +1,4 @@
+using System.Globalization;
 using System.Text.Json.Serialization;
 
 namespace AzureEventGridSimulator.Domain.Entities;
@@ -76,10 +77,22 @@ public class CloudEvent
     public string DataBase64 { get; set; }
 
     [JsonIgnore]
-    private DateTime? TimeParsed => string.IsNullOrEmpty(Time) ? null : DateTime.Parse(Time);
+    private DateTimeOffset? TimeParsed =>
+        string.IsNullOrEmpty(Time)
+            ? null
+            : DateTimeOffset.Parse(Time, CultureInfo.InvariantCulture);
 
     [JsonIgnore]
-    private bool TimeIsValid => string.IsNullOrEmpty(Time) || DateTime.TryParse(Time, out _);
+    private bool TimeIsValid =>
+        string.IsNullOrEmpty(Time)
+        || DateTimeOffset.TryParse(Time, CultureInfo.InvariantCulture, out _);
+
+    [JsonIgnore]
+    private bool TimeHasTimezone =>
+        string.IsNullOrEmpty(Time)
+        || Time.Contains('Z')
+        || Time.Contains('+')
+        || (Time.Length > 10 && Time[10..].Contains('-'));
 
     /// <summary>
     /// Validate the CloudEvent according to the CloudEvents v1.0 specification.
@@ -141,10 +154,10 @@ public void Validate()
                 );
             }
 
-            if (TimeParsed is { Kind: DateTimeKind.Unspecified })
+            if (!TimeHasTimezone)
             {
                 throw new InvalidOperationException(
-                    $"Property '{nameof(Time)}' must include timezone information."
+                    $"Property '{nameof(Time)}' must include timezone information (e.g., 'Z' for UTC or an offset like '+00:00')."
                 );
             }
         }
diff --git a/src/AzureEventGridSimulator/Domain/Entities/DeadLetterEvent.cs b/src/AzureEventGridSimulator/Domain/Entities/DeadLetterEvent.cs
index 0b22cbd..8e2d654 100644
--- a/src/AzureEventGridSimulator/Domain/Entities/DeadLetterEvent.cs
+++ b/src/AzureEventGridSimulator/Domain/Entities/DeadLetterEvent.cs
@@ -44,13 +44,13 @@ public class DeadLetterEvent
     /// Gets or sets the time the event was originally published.
     /// </summary>
     [JsonPropertyName("publishTime")]
-    public DateTime PublishTime { get; init; }
+    public DateTimeOffset PublishTime { get; init; }
 
     /// <summary>
     /// Gets or sets the time of the last delivery attempt.
     /// </summary>
     [JsonPropertyName("lastDeliveryAttemptTime")]
-    public DateTime LastDeliveryAttemptTime { get; init; }
+    public DateTimeOffset LastDeliveryAttemptTime { get; init; }
 
     /// <summary>
     /// Gets or sets the topic name.
diff --git a/src/AzureEventGridSimulator/Domain/Entities/DeliveryAttempt.cs b/src/AzureEventGridSimulator/Domain/Entities/DeliveryAttempt.cs
index 6eca0d7..ab1da10 100644
--- a/src/AzureEventGridSimulator/Domain/Entities/DeliveryAttempt.cs
+++ b/src/AzureEventGridSimulator/Domain/Entities/DeliveryAttempt.cs
@@ -8,7 +8,7 @@ public class DeliveryAttempt
     /// <summary>
     /// Gets or sets the time of the attempt.
     /// </summary>
-    public DateTime AttemptTime { get; init; } = DateTime.UtcNow;
+    public DateTimeOffset AttemptTime { get; init; } = DateTimeOffset.UtcNow;
 
     /// <summary>
     /// Gets or sets the attempt number (1-based).
diff --git a/src/AzureEventGridSimulator/Domain/Entities/EventGridEvent.cs b/src/AzureEventGridSimulator/Domain/Entities/EventGridEvent.cs
index f719aa1..7486d68 100644
--- a/src/AzureEventGridSimulator/Domain/Entities/EventGridEvent.cs
+++ b/src/AzureEventGridSimulator/Domain/Entities/EventGridEvent.cs
@@ -1,3 +1,4 @@
+using System.Globalization;
 using System.Text.Json.Serialization;
 
 namespace AzureEventGridSimulator.Domain.Entities;
@@ -38,10 +39,18 @@ public class EventGridEvent
     public string EventTime { get; set; }
 
     [JsonIgnore]
-    private DateTime EventTimeParsed => DateTime.Parse(EventTime);
+    private DateTimeOffset EventTimeParsed =>
+        DateTimeOffset.Parse(EventTime, CultureInfo.InvariantCulture);
 
     [JsonIgnore]
-    private bool EventTimeIsValid => DateTime.TryParse(EventTime, out _);
+    private bool EventTimeIsValid =>
+        DateTimeOffset.TryParse(EventTime, CultureInfo.InvariantCulture, out _);
+
+    [JsonIgnore]
+    private bool EventTimeHasTimezone =>
+        EventTime.Contains('Z')
+        || EventTime.Contains('+')
+        || (EventTime.Length > 10 && EventTime[10..].Contains('-'));
 
     /// <summary>
     /// Gets or sets the schema version of the data object.
@@ -56,10 +65,27 @@ public class EventGridEvent
     public string MetadataVersion { get; set; }
 
     /// <summary>
-    /// Gets or sets the resource path of the event source.
+    /// Gets the resource path of the event source.
+    /// This property is set by Event Grid, not by publishers.
     /// </summary>
     [JsonPropertyName("topic")]
-    public string Topic { get; set; }
+    [JsonInclude]
+    public string Topic { get; private set; }
+
+    /// <summary>
+    /// Indicates whether the Topic has been set by the simulator.
+    /// </summary>
+    [JsonIgnore]
+    internal bool TopicHasBeenSet { get; private set; }
+
+    /// <summary>
+    /// Sets the topic path. This should only be called by the simulator.
+    /// </summary>
+    internal void SetTopic(string topic)
+    {
+        Topic = topic;
+        TopicHasBeenSet = true;
+    }
 
     /// <summary>
     /// Validate the object.
@@ -102,10 +128,10 @@ public void Validate()
             );
         }
 
-        if (EventTimeParsed.Kind == DateTimeKind.Unspecified)
+        if (!EventTimeHasTimezone)
         {
             throw new InvalidOperationException(
-                $"Property '{nameof(EventTime)}' must be either Local or UTC."
+                $"Property '{nameof(EventTime)}' must include a timezone indicator (e.g., 'Z' for UTC or an offset like '+00:00')."
             );
         }
 
@@ -116,7 +142,9 @@ public void Validate()
             );
         }
 
-        if (!string.IsNullOrEmpty(Topic))
+        // Topic must NOT be set by the publisher - Event Grid sets this automatically
+        // Skip this check if the simulator has already set the topic via SetTopic()
+        if (!TopicHasBeenSet && !string.IsNullOrEmpty(Topic))
         {
             throw new InvalidOperationException(
                 $"Property '{nameof(Topic)}' was found to be set to '{Topic}', but was expected to either be null/empty."
diff --git a/src/AzureEventGridSimulator/Domain/Entities/PendingDelivery.cs b/src/AzureEventGridSimulator/Domain/Entities/PendingDelivery.cs
index 0bb6c53..b3112b3 100644
--- a/src/AzureEventGridSimulator/Domain/Entities/PendingDelivery.cs
+++ b/src/AzureEventGridSimulator/Domain/Entities/PendingDelivery.cs
@@ -36,12 +36,12 @@ public class PendingDelivery
     /// <summary>
     /// Gets or sets the time the event was enqueued.
     /// </summary>
-    public DateTime EnqueuedTime { get; init; } = DateTime.UtcNow;
+    public DateTimeOffset EnqueuedTime { get; init; } = DateTimeOffset.UtcNow;
 
     /// <summary>
     /// Gets or sets the next scheduled attempt time.
     /// </summary>
-    public DateTime NextAttemptTime { get; set; } = DateTime.UtcNow;
+    public DateTimeOffset NextAttemptTime { get; set; } = DateTimeOffset.UtcNow;
 
     /// <summary>
     /// Gets or sets the number of delivery attempts made.
@@ -59,12 +59,6 @@ public class PendingDelivery
     private RetryPolicySettings EffectiveRetryPolicy =>
         Subscriber.RetryPolicy ?? new RetryPolicySettings();
 
-    /// <summary>
-    /// Gets whether the event has expired based on TTL.
-    /// </summary>
-    public bool IsExpired =>
-        DateTime.UtcNow > EnqueuedTime.AddMinutes(EffectiveRetryPolicy.EventTimeToLiveInMinutes);
-
     /// <summary>
     /// Gets whether the maximum delivery attempts have been reached.
     /// </summary>
@@ -79,4 +73,18 @@ public class PendingDelivery
     /// Gets the last delivery attempt, if any.
     /// </summary>
     public DeliveryAttempt LastAttempt => Attempts.Count > 0 ? Attempts[^1] : null;
+
+    /// <summary>
+    /// Determines whether the event has expired based on TTL.
+    /// </summary>
+    /// <param name="now" >
+    /// The current UTC time.
+    /// </param>
+    /// <returns>
+    /// True if the event has expired.
+    /// </returns>
+    public bool IsExpired(DateTimeOffset now)
+    {
+        return now > EnqueuedTime.AddMinutes(EffectiveRetryPolicy.EventTimeToLiveInMinutes);
+    }
 }
diff --git a/src/AzureEventGridSimulator/Domain/Services/Delivery/DeliveryPropertyResolver.cs b/src/AzureEventGridSimulator/Domain/Services/Delivery/DeliveryPropertyResolver.cs
index d8ae119..1d4a718 100644
--- a/src/AzureEventGridSimulator/Domain/Services/Delivery/DeliveryPropertyResolver.cs
+++ b/src/AzureEventGridSimulator/Domain/Services/Delivery/DeliveryPropertyResolver.cs
@@ -1,3 +1,4 @@
+using System.Globalization;
 using System.Text.Json;
 using AzureEventGridSimulator.Domain.Entities;
 using AzureEventGridSimulator.Infrastructure.Settings.Subscribers;
@@ -202,7 +203,7 @@ JsonValueKind.Number when element.TryGetInt64(out var l) => l,
         };
     }
 
-    private static bool TryParseDateTime(string value, out DateTime result)
+    private static bool TryParseDateTime(string value, out DateTimeOffset result)
     {
         result = default;
         if (string.IsNullOrEmpty(value))
@@ -210,7 +211,7 @@ private static bool TryParseDateTime(string value, out DateTime result)
             return false;
         }
 
-        return DateTime.TryParse(value, out result);
+        return DateTimeOffset.TryParse(value, CultureInfo.InvariantCulture, out result);
     }
 
     private static bool TryParseGuid(string value, out Guid result)
diff --git a/src/AzureEventGridSimulator/Domain/Services/EventGridSchemaFormatter.cs b/src/AzureEventGridSimulator/Domain/Services/EventGridSchemaFormatter.cs
index e02bc60..1083499 100644
--- a/src/AzureEventGridSimulator/Domain/Services/EventGridSchemaFormatter.cs
+++ b/src/AzureEventGridSimulator/Domain/Services/EventGridSchemaFormatter.cs
@@ -6,7 +6,7 @@ namespace AzureEventGridSimulator.Domain.Services;
 /// <summary>
 /// Formats events in the Azure Event Grid schema for delivery.
 /// </summary>
-public class EventGridSchemaFormatter : IEventSchemaFormatter
+public class EventGridSchemaFormatter(TimeProvider timeProvider) : IEventSchemaFormatter
 {
     /// <inheritdoc />
     public EventSchema Schema => EventSchema.EventGridSchema;
@@ -62,17 +62,18 @@ private EventGridEvent ConvertToEventGridEvent(SimulatorEvent evt)
     /// </summary>
     private EventGridEvent ConvertCloudEventToEventGrid(CloudEvent cloudEvent)
     {
-        return new EventGridEvent
+        var eventGridEvent = new EventGridEvent
         {
             Id = cloudEvent.Id,
             Subject = cloudEvent.Subject ?? cloudEvent.Source,
             EventType = cloudEvent.Type,
-            EventTime = cloudEvent.Time ?? DateTime.UtcNow.ToString("o"),
+            EventTime = cloudEvent.Time ?? timeProvider.GetUtcNow().ToString("o"),
             Data = cloudEvent.Data,
             DataVersion = ExtractDataVersion(cloudEvent.DataSchema),
-            Topic = cloudEvent.Source,
             MetadataVersion = "1",
         };
+        eventGridEvent.SetTopic(cloudEvent.Source);
+        return eventGridEvent;
     }
 
     /// <summary>
diff --git a/src/AzureEventGridSimulator/Domain/Services/Retry/InMemoryDeliveryQueue.cs b/src/AzureEventGridSimulator/Domain/Services/Retry/InMemoryDeliveryQueue.cs
index 11dc997..82b49b4 100644
--- a/src/AzureEventGridSimulator/Domain/Services/Retry/InMemoryDeliveryQueue.cs
+++ b/src/AzureEventGridSimulator/Domain/Services/Retry/InMemoryDeliveryQueue.cs
@@ -7,7 +7,8 @@ namespace AzureEventGridSimulator.Domain.Services.Retry;
 /// Thread-safe in-memory implementation of the delivery queue.
 /// Events are lost on restart (acceptable for a simulator).
 /// </summary>
-public class InMemoryDeliveryQueue(ILogger<InMemoryDeliveryQueue> logger) : IDeliveryQueue
+public class InMemoryDeliveryQueue(TimeProvider timeProvider, ILogger<InMemoryDeliveryQueue> logger)
+    : IDeliveryQueue
 {
     private readonly ConcurrentDictionary<string, PendingDelivery> _queue = new();
 
@@ -55,7 +56,7 @@ public void RequeueForRetry(PendingDelivery delivery)
     /// <inheritdoc />
     public IEnumerable<PendingDelivery> GetDueDeliveries()
     {
-        var now = DateTime.UtcNow;
+        var now = timeProvider.GetUtcNow();
 
         return _queue
             .Values.Where(d => d.NextAttemptTime <= now)
diff --git a/src/AzureEventGridSimulator/Domain/Services/Retry/RetryDeliveryBackgroundService.cs b/src/AzureEventGridSimulator/Domain/Services/Retry/RetryDeliveryBackgroundService.cs
index b054f11..eae4438 100644
--- a/src/AzureEventGridSimulator/Domain/Services/Retry/RetryDeliveryBackgroundService.cs
+++ b/src/AzureEventGridSimulator/Domain/Services/Retry/RetryDeliveryBackgroundService.cs
@@ -14,6 +14,8 @@ public class RetryDeliveryBackgroundService(
     IServiceProvider serviceProvider,
     DeadLetterService deadLetterService,
     IEventHistoryService eventHistoryService,
+    RetryScheduler retryScheduler,
+    TimeProvider timeProvider,
     ILogger<RetryDeliveryBackgroundService> logger
 ) : BackgroundService
 {
@@ -84,7 +86,7 @@ CancellationToken cancellationToken
     )
     {
         // Check if event has expired (TTL)
-        if (delivery.IsExpired)
+        if (delivery.IsExpired(timeProvider.GetUtcNow()))
         {
             logger.LogWarning(
                 "Event {EventId} expired for subscriber '{SubscriberName}'. TTL exceeded",
@@ -97,7 +99,7 @@ CancellationToken cancellationToken
                 delivery.Event.Id,
                 delivery.Subscriber.Name,
                 DeliveryStatus.DeadLettered,
-                DateTimeOffset.UtcNow
+                timeProvider.GetUtcNow()
             );
             return;
         }
@@ -117,7 +119,7 @@ CancellationToken cancellationToken
                 delivery.Event.Id,
                 delivery.Subscriber.Name,
                 DeliveryStatus.DeadLettered,
-                DateTimeOffset.UtcNow
+                timeProvider.GetUtcNow()
             );
             return;
         }
@@ -129,7 +131,7 @@ CancellationToken cancellationToken
         delivery.AttemptCount++;
         var attempt = new DeliveryAttempt
         {
-            AttemptTime = DateTime.UtcNow,
+            AttemptTime = timeProvider.GetUtcNow(),
             AttemptNumber = delivery.AttemptCount,
             Outcome = result.Outcome,
             HttpStatusCode = result.HttpStatusCode,
@@ -156,7 +158,7 @@ CancellationToken cancellationToken
                 delivery.Event.Id,
                 delivery.Subscriber.Name,
                 DeliveryStatus.Delivered,
-                DateTimeOffset.UtcNow
+                timeProvider.GetUtcNow()
             );
             return;
         }
@@ -315,10 +317,10 @@ CancellationToken cancellationToken
         // Check for immediate dead-letter conditions (HTTP 400, 401, 403, 413)
         if (
             result.HttpStatusCode.HasValue
-            && RetryScheduler.ShouldImmediatelyDeadLetter(result.HttpStatusCode.Value)
+            && retryScheduler.ShouldImmediatelyDeadLetter(result.HttpStatusCode.Value)
         )
         {
-            var reason = RetryScheduler.GetDeadLetterReasonForStatusCode(
+            var reason = retryScheduler.GetDeadLetterReasonForStatusCode(
                 result.HttpStatusCode.Value
             );
 
@@ -334,7 +336,7 @@ CancellationToken cancellationToken
                 delivery.Event.Id,
                 delivery.Subscriber.Name,
                 DeliveryStatus.DeadLettered,
-                DateTimeOffset.UtcNow
+                timeProvider.GetUtcNow()
             );
             return;
         }
@@ -353,7 +355,7 @@ CancellationToken cancellationToken
                 delivery.Event.Id,
                 delivery.Subscriber.Name,
                 DeliveryStatus.DeadLettered,
-                DateTimeOffset.UtcNow
+                timeProvider.GetUtcNow()
             );
             return;
         }
@@ -372,13 +374,13 @@ CancellationToken cancellationToken
                 delivery.Event.Id,
                 delivery.Subscriber.Name,
                 DeliveryStatus.DeadLettered,
-                DateTimeOffset.UtcNow
+                timeProvider.GetUtcNow()
             );
             return;
         }
 
         // Check if TTL will be exceeded before next retry
-        if (delivery.IsExpired)
+        if (delivery.IsExpired(timeProvider.GetUtcNow()))
         {
             logger.LogWarning(
                 "Event {EventId} TTL expired during retry for '{SubscriberName}'",
@@ -391,13 +393,13 @@ CancellationToken cancellationToken
                 delivery.Event.Id,
                 delivery.Subscriber.Name,
                 DeliveryStatus.DeadLettered,
-                DateTimeOffset.UtcNow
+                timeProvider.GetUtcNow()
             );
             return;
         }
 
         // Schedule retry
-        delivery.NextAttemptTime = RetryScheduler.GetNextRetryTime(
+        delivery.NextAttemptTime = retryScheduler.GetNextRetryTime(
             delivery.AttemptCount,
             result.HttpStatusCode
         );
diff --git a/src/AzureEventGridSimulator/Domain/Services/Retry/RetryScheduler.cs b/src/AzureEventGridSimulator/Domain/Services/Retry/RetryScheduler.cs
index e93f56e..5fb8dbf 100644
--- a/src/AzureEventGridSimulator/Domain/Services/Retry/RetryScheduler.cs
+++ b/src/AzureEventGridSimulator/Domain/Services/Retry/RetryScheduler.cs
@@ -3,7 +3,7 @@ namespace AzureEventGridSimulator.Domain.Services.Retry;
 /// <summary>
 /// Calculates next retry time based on Azure Event Grid retry schedule.
 /// </summary>
-public static class RetryScheduler
+public class RetryScheduler
 {
     /// <summary>
     /// Azure Event Grid retry schedule with exponential backoff.
@@ -32,6 +32,13 @@ public static class RetryScheduler
     /// </summary>
     private static readonly int[] ImmediateDeadLetterStatusCodes = [400, 401, 403, 413];
 
+    private readonly TimeProvider _timeProvider;
+
+    public RetryScheduler(TimeProvider timeProvider)
+    {
+        _timeProvider = timeProvider;
+    }
+
     /// <summary>
     /// Calculates the next retry time based on attempt number and HTTP status code.
     /// </summary>
@@ -44,10 +51,10 @@ public static class RetryScheduler
     /// <returns>
     /// The next retry time.
     /// </returns>
-    public static DateTime GetNextRetryTime(int attemptNumber, int? httpStatusCode = null)
+    public DateTimeOffset GetNextRetryTime(int attemptNumber, int? httpStatusCode = null)
     {
         var delay = GetRetryDelay(attemptNumber, httpStatusCode);
-        return DateTime.UtcNow.Add(delay);
+        return _timeProvider.GetUtcNow().Add(delay);
     }
 
     /// <summary>
@@ -101,7 +108,7 @@ private static TimeSpan GetStandardDelay(int attemptNumber)
     /// <returns>
     /// True if the status code indicates success.
     /// </returns>
-    public static bool IsSuccessStatusCode(int statusCode)
+    public bool IsSuccessStatusCode(int statusCode)
     {
         return SuccessStatusCodes.Contains(statusCode);
     }
@@ -115,7 +122,7 @@ public static bool IsSuccessStatusCode(int statusCode)
     /// <returns>
     /// True if the event should be immediately dead-lettered.
     /// </returns>
-    public static bool ShouldImmediatelyDeadLetter(int statusCode)
+    public bool ShouldImmediatelyDeadLetter(int statusCode)
     {
         return ImmediateDeadLetterStatusCodes.Contains(statusCode);
     }
@@ -129,7 +136,7 @@ public static bool ShouldImmediatelyDeadLetter(int statusCode)
     /// <returns>
     /// The dead-letter reason string.
     /// </returns>
-    public static string GetDeadLetterReasonForStatusCode(int statusCode)
+    public string GetDeadLetterReasonForStatusCode(int statusCode)
     {
         return statusCode switch
         {
diff --git a/src/AzureEventGridSimulator/Infrastructure/Middleware/SasKeyValidator.cs b/src/AzureEventGridSimulator/Infrastructure/Middleware/SasKeyValidator.cs
index 128ddd2..dbbccc8 100644
--- a/src/AzureEventGridSimulator/Infrastructure/Middleware/SasKeyValidator.cs
+++ b/src/AzureEventGridSimulator/Infrastructure/Middleware/SasKeyValidator.cs
@@ -1,4 +1,5 @@
-using System.Security.Cryptography;
+using System.Globalization;
+using System.Security.Cryptography;
 using System.Text;
 using System.Web;
 using AzureEventGridSimulator.Domain;
@@ -6,7 +7,7 @@
 
 namespace AzureEventGridSimulator.Infrastructure.Middleware;
 
-public class SasKeyValidator(ILogger<SasKeyValidator> logger)
+public class SasKeyValidator(TimeProvider timeProvider, ILogger<SasKeyValidator> logger)
 {
     public bool IsValid(IHeaderDictionary requestHeaders, string topicKey)
     {
@@ -77,8 +78,12 @@ private bool TokenIsValid(string token, string key)
         var encodedSignature = query["s"];
 
         if (
-            !DateTime.TryParse(decodedExpiration, out var tokenExpiryDateTime)
-            || tokenExpiryDateTime.ToUniversalTime() <= DateTime.UtcNow
+            !DateTimeOffset.TryParse(
+                decodedExpiration,
+                CultureInfo.InvariantCulture,
+                out var tokenExpiryDateTime
+            )
+            || tokenExpiryDateTime.ToUniversalTime() <= timeProvider.GetUtcNow()
         )
         {
             return false;
diff --git a/src/AzureEventGridSimulator/Infrastructure/Settings/Subscribers/HttpSubscriberSettings.cs b/src/AzureEventGridSimulator/Infrastructure/Settings/Subscribers/HttpSubscriberSettings.cs
index 4f61fb0..9231f5a 100644
--- a/src/AzureEventGridSimulator/Infrastructure/Settings/Subscribers/HttpSubscriberSettings.cs
+++ b/src/AzureEventGridSimulator/Infrastructure/Settings/Subscribers/HttpSubscriberSettings.cs
@@ -9,7 +9,7 @@ namespace AzureEventGridSimulator.Infrastructure.Settings.Subscribers;
 /// </summary>
 public class HttpSubscriberSettings : ISubscriberSettings
 {
-    private readonly DateTime _expired = DateTime.UtcNow.AddMinutes(5);
+    private readonly DateTimeOffset _createdAt = DateTimeOffset.UtcNow;
 
     [JsonPropertyName("endpoint")]
     public string Endpoint { get; set; }
@@ -23,9 +23,6 @@ public class HttpSubscriberSettings : ISubscriberSettings
     [JsonIgnore]
     public Guid ValidationCode => GetValidationCode();
 
-    [JsonIgnore]
-    public bool ValidationPeriodExpired => DateTime.UtcNow > _expired;
-
     [JsonPropertyName("name")]
     public string Name { get; set; }
 
@@ -91,6 +88,20 @@ public void Validate()
         DeadLetter?.Validate();
     }
 
+    /// <summary>
+    /// Determines if the validation period has expired.
+    /// </summary>
+    /// <param name="now" >
+    /// The current UTC time.
+    /// </param>
+    /// <returns>
+    /// True if the 5-minute validation window has expired.
+    /// </returns>
+    public bool ValidationPeriodExpired(DateTimeOffset now)
+    {
+        return now > _createdAt.AddMinutes(5);
+    }
+
     public Guid GetValidationCode()
     {
         return new Guid(
diff --git a/src/AzureEventGridSimulator/Infrastructure/Settings/SubscriptionSettings.cs b/src/AzureEventGridSimulator/Infrastructure/Settings/SubscriptionSettings.cs
index 7df2cc7..b897d85 100644
--- a/src/AzureEventGridSimulator/Infrastructure/Settings/SubscriptionSettings.cs
+++ b/src/AzureEventGridSimulator/Infrastructure/Settings/SubscriptionSettings.cs
@@ -6,7 +6,7 @@ namespace AzureEventGridSimulator.Infrastructure.Settings;
 
 public class SubscriptionSettings
 {
-    private readonly DateTime _expired = DateTime.UtcNow.AddMinutes(5);
+    private readonly DateTimeOffset _createdAt = DateTimeOffset.UtcNow;
 
     [JsonPropertyName("name")]
     public string Name { get; set; }
@@ -37,8 +37,19 @@ public class SubscriptionSettings
     [JsonIgnore]
     public Guid ValidationCode => GetValidationCode();
 
-    [JsonIgnore]
-    public bool ValidationPeriodExpired => DateTime.UtcNow > _expired;
+    /// <summary>
+    /// Determines if the validation period has expired.
+    /// </summary>
+    /// <param name="now" >
+    /// The current UTC time.
+    /// </param>
+    /// <returns>
+    /// True if the 5-minute validation window has expired.
+    /// </returns>
+    public bool ValidationPeriodExpired(DateTimeOffset now)
+    {
+        return now > _createdAt.AddMinutes(5);
+    }
 
     public Guid GetValidationCode()
     {
diff --git a/src/AzureEventGridSimulator/Program.cs b/src/AzureEventGridSimulator/Program.cs
index 6257b67..3e48d80 100644
--- a/src/AzureEventGridSimulator/Program.cs
+++ b/src/AzureEventGridSimulator/Program.cs
@@ -279,6 +279,9 @@ IConfiguration configuration
         builder.Services.AddSimulatorSettings(configuration);
         builder.Services.AddMediator(Assembly.GetExecutingAssembly());
 
+        // Register TimeProvider for testability
+        builder.Services.AddSingleton(TimeProvider.System);
+
         // Register event schema services (stateless, can be singletons)
         builder.Services.AddSingleton<EventSchemaDetector>();
         builder.Services.AddSingleton<EventGridSchemaParser>();
@@ -296,6 +299,7 @@ IConfiguration configuration
         builder.Services.AddSingleton<HttpEventDeliveryService>();
 
         // Register retry and dead-letter services
+        builder.Services.AddSingleton<RetryScheduler>();
         builder.Services.AddSingleton<IDeliveryQueue, InMemoryDeliveryQueue>();
         builder.Services.AddSingleton<DeadLetterService>();
         builder.Services.AddHostedService<RetryDeliveryBackgroundService>();