fix: CalDateTime CTOR using ISO 8601 UTC string resolves to UTC

Throws for controversion of ISO 8601 UTC string, while timezone ID is not UTC

chore: `ExcludeFromCodeCoverage` for private constructor
chore: Remove unused method `TruncateTimeToSeconds`

Resolves #831

Author: axunonb

Ical.Net.Tests/CalDateTimeTests.cs

@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using Ical.Net.CalendarComponents;
 using Ical.Net.DataTypes;
 using NUnit.Framework;
@@ -55,7 +56,7 @@ public void ToTimeZoneFloating()
     [Test, TestCaseSource(nameof(ToTimeZoneTestCases))]
     public void ToTimeZoneTests(CalendarEvent calendarEvent, string targetTimeZone)
     {
-        var startAsUtc = calendarEvent.Start.AsUtc;
+        var startAsUtc = calendarEvent.Start!.AsUtc;
 
         var convertedStart = calendarEvent.Start.ToTimeZone(targetTimeZone);
         var convertedAsUtc = convertedStart.AsUtc;
@@ -369,4 +370,23 @@ public void CalDateTime_FromDateTime_HandlesKindCorrectly(DateTimeKind kind, IRe
 
         Assert.That(() => new CalDateTime(dt), constraint);
     }
+
+    [TestCase("20250703T060000Z", null)]
+    [TestCase("20250703T060000Z", CalDateTime.UtcTzId)]
+    public void ConstructorWithIso8601UtcString_ShouldResultInUtc(string value, string? tzId)
+    {
+        var dt = new CalDateTime(value, tzId);
+        Assert.Multiple(() =>
+        {
+            Assert.That(dt.Value, Is.EqualTo(new DateTime(2025, 7, 3, 6, 0, 0, DateTimeKind.Utc)));
+#pragma warning disable CA1305
+            Assert.That(dt.ToString("yyyy-MM-dd HH:mm:ss"), Is.EqualTo("2025-07-03 06:00:00 UTC"));
+#pragma warning restore CA1305
+            Assert.That(dt.IsUtc, Is.True);
+        });
+    }
+
+    [Test]
+    public void ConstructorWithIso8601UtcString_ButDifferentTzId_ShouldThrow()
+        => Assert.That(() => _ = new CalDateTime("20250703T060000Z", "CEST"), Throws.ArgumentException);
 }

Ical.Net/DataTypes/CalDateTime.cs

@@ -7,6 +7,7 @@
 using Ical.Net.Utility;
 using NodaTime;
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.IO;
 
@@ -55,6 +56,7 @@ public sealed class CalDateTime : IComparable<CalDateTime>, IFormattable
     /// <summary>
     /// This constructor is required for the SerializerFactory to work.
     /// </summary>
+    [ExcludeFromCodeCoverage]
     private CalDateTime()
     {
         // required for the SerializerFactory to work
@@ -201,9 +203,15 @@ public CalDateTime(string value, string? tzId = null)
     {
         var serializer = new DateTimeSerializer();
         CopyFrom(serializer.Deserialize(new StringReader(value)) as CalDateTime
-                 ?? throw new InvalidOperationException($"$Failure for deserializing value '{value}'"));
+                 ?? throw new InvalidOperationException($"$Failure when deserializing value '{value}'"));
+
         // The string may contain a date only, meaning that the tzId should be ignored.
-        _tzId = HasTime ? tzId : null;
+        _tzId ??= HasTime ? tzId : null;
+
+        if (IsUtc && tzId != null && !string.Equals(tzId, UtcTzId, StringComparison.OrdinalIgnoreCase))
+        {
+            throw new ArgumentException($"The value '{value}' is a UTC date/time, but the specified timezone '{tzId}' is not '{UtcTzId}'.", nameof(tzId));
+        }
     }
 
     private void Initialize(DateOnly dateOnly, TimeOnly? timeOnly, string? tzId)
@@ -218,7 +226,6 @@ private void Initialize(DateOnly dateOnly, TimeOnly? timeOnly, string? tzId)
         };
     }
 
-    /// <inheritdoc/>
     private void CopyFrom(CalDateTime calDt)
     {
         // Maintain the private date/time backing fields
@@ -436,12 +443,6 @@ public DateTime Value
         return new TimeOnly(time.Value.Hour, time.Value.Minute, time.Value.Second);
     }
 
-    /// <summary>
-    /// Any <see cref="Time"/> values are truncated to seconds, because
-    /// RFC 5545, Section 3.3.5 does not allow for fractional seconds.
-    /// </summary>
-    private static TimeOnly? TruncateTimeToSeconds(DateTime dateTime) => new TimeOnly(dateTime.Hour, dateTime.Minute, dateTime.Second);
-
     /// <summary>
     /// Converts the <see cref="Value"/> to a date/time
     /// within the specified <see paramref="otherTzId"/> timezone.