diff --git a/tensorboard/data/server/BUILD b/tensorboard/data/server/BUILD
index 5c2387adde..32694283ef 100644
--- a/tensorboard/data/server/BUILD
+++ b/tensorboard/data/server/BUILD
@@ -32,6 +32,7 @@ rust_library(
         "reservoir.rs",
         "scripted_reader.rs",
         "tf_record.rs",
+        "types.rs",
     ] + _checked_in_proto_files,
     edition = "2018",
     deps = [
diff --git a/tensorboard/data/server/lib.rs b/tensorboard/data/server/lib.rs
index 1230ddfe09..fdfd4c2d65 100644
--- a/tensorboard/data/server/lib.rs
+++ b/tensorboard/data/server/lib.rs
@@ -19,6 +19,7 @@ pub mod event_file;
 pub mod masked_crc;
 pub mod reservoir;
 pub mod tf_record;
+pub mod types;
 
 #[cfg(test)]
 mod scripted_reader;
diff --git a/tensorboard/data/server/reservoir.rs b/tensorboard/data/server/reservoir.rs
index 0d4dd3f62b..ba8131ad24 100644
--- a/tensorboard/data/server/reservoir.rs
+++ b/tensorboard/data/server/reservoir.rs
@@ -21,6 +21,8 @@ use rand::{
 };
 use rand_chacha::ChaCha20Rng;
 
+use crate::types::Step;
+
 /// A [reservoir sampling] data structure, with support for preemption and deferred "commits" of
 /// records to a separate destination for better concurrency.
 ///
@@ -98,10 +100,6 @@ pub struct StageReservoir<T, C = ChaCha20Rng> {
     seen: usize,
 }
 
-/// A step associated with a record, strictly increasing over time within a record stream.
-#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Copy, Clone)]
-pub struct Step(pub i64);
-
 /// A buffer of records that have been committed and not yet evicted from the reservoir.
 ///
 /// This is a snapshot of the reservoir contents at some point in time that is periodically updated
diff --git a/tensorboard/data/server/types.rs b/tensorboard/data/server/types.rs
new file mode 100644
index 0000000000..59a764504a
--- /dev/null
+++ b/tensorboard/data/server/types.rs
@@ -0,0 +1,110 @@
+/* Copyright 2020 The TensorFlow Authors. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+==============================================================================*/
+
+//! Core simple types.
+
+use std::borrow::Borrow;
+
+/// A step associated with a record, strictly increasing over time within a record stream.
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Copy, Clone)]
+pub struct Step(pub i64);
+
+/// The wall time of a TensorBoard event.
+///
+/// Wall times represent floating-point seconds since Unix epoch. They must be finite and non-NaN.
+#[derive(Debug, PartialEq, PartialOrd, Copy, Clone)]
+pub struct WallTime(f64);
+
+impl WallTime {
+    /// Parses a wall time from a time stamp representing seconds since Unix epoch.
+    ///
+    /// Returns `None` if the given time is infinite or NaN.
+    pub fn new(time: f64) -> Option<Self> {
+        if time.is_finite() {
+            Some(WallTime(time))
+        } else {
+            None
+        }
+    }
+}
+
+// Wall times are totally ordered and have a total equivalence relation, since we guarantee that
+// they are not NaN.
+#[allow(clippy::derive_ord_xor_partial_ord)] // okay because it agrees with `PartialOrd` impl
+impl Ord for WallTime {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.partial_cmp(&other)
+            .unwrap_or_else(|| unreachable!("{:?} <> {:?}", &self, &other))
+    }
+}
+impl Eq for WallTime {}
+
+impl From<WallTime> for f64 {
+    fn from(wt: WallTime) -> f64 {
+        wt.0
+    }
+}
+
+/// The name of a time series within the context of a run.
+///
+/// Tag names are valid Unicode text strings. They should be non-empty, though this type does not
+/// enforce that.
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone)]
+pub struct Tag(pub String);
+
+impl Borrow<str> for Tag {
+    fn borrow(&self) -> &str {
+        &self.0
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_tag_hash_map_str_access() {
+        use std::collections::HashMap;
+        let mut m: HashMap<Tag, i32> = HashMap::new();
+        m.insert(Tag("accuracy".to_string()), 1);
+        m.insert(Tag("loss".to_string()), 2);
+        // We can call `get` given only a `&str`, not an owned `Tag`.
+        assert_eq!(m.get("accuracy"), Some(&1));
+        assert_eq!(m.get("xent"), None);
+    }
+
+    #[test]
+    fn test_wall_time() {
+        assert_eq!(WallTime::new(f64::INFINITY), None);
+        assert_eq!(WallTime::new(-f64::INFINITY), None);
+        assert_eq!(WallTime::new(f64::NAN), None);
+
+        assert_eq!(f64::from(WallTime::new(1234.5).unwrap()), 1234.5);
+        assert!(WallTime::new(1234.5) < WallTime::new(2345.625));
+
+        let mut actual = vec![
+            WallTime::new(123.0).unwrap(),
+            WallTime::new(-456.0).unwrap(),
+            WallTime::new(789.0).unwrap(),
+        ];
+        actual.sort();
+        let expected = vec![
+            WallTime::new(-456.0).unwrap(),
+            WallTime::new(123.0).unwrap(),
+            WallTime::new(789.0).unwrap(),
+        ];
+        assert_eq!(actual, expected);
+    }
+}