diff --git a/crates/ruma-common/src/serde/duration.rs b/crates/ruma-common/src/serde/duration.rs
index 4f8de31b..13434340 100644
--- a/crates/ruma-common/src/serde/duration.rs
+++ b/crates/ruma-common/src/serde/duration.rs
@@ -1,5 +1,6 @@
 //! De-/serialization functions for `std::time::Duration` objects
 
+pub mod ms;
 pub mod opt_ms;
 pub mod opt_secs;
 pub mod secs;
diff --git a/crates/ruma-common/src/serde/duration/ms.rs b/crates/ruma-common/src/serde/duration/ms.rs
new file mode 100644
index 00000000..e0e87595
--- /dev/null
+++ b/crates/ruma-common/src/serde/duration/ms.rs
@@ -0,0 +1,67 @@
+//! De-/serialization functions for `Option<std::time::Duration>` objects represented as
+//! milliseconds.
+//!
+//! Delegates to `js_int::UInt` to ensure integer size is within bounds.
+
+use std::time::Duration;
+
+use js_int::UInt;
+use serde::{
+    de::{Deserialize, Deserializer},
+    ser::{Error, Serialize, Serializer},
+};
+
+/// Serializes a Duration to an integer representing seconds.
+///
+/// Will fail if integer is greater than the maximum integer that can be
+/// unambiguously represented by an f64.
+pub fn serialize<S>(duration: &Duration, serializer: S) -> Result<S::Ok, S::Error>
+where
+    S: Serializer,
+{
+    match UInt::try_from(duration.as_millis()) {
+        Ok(uint) => uint.serialize(serializer),
+        Err(err) => Err(S::Error::custom(err)),
+    }
+}
+
+/// Deserializes an integer representing seconds into a Duration.
+///
+/// Will fail if integer is greater than the maximum integer that can be
+/// unambiguously represented by an f64.
+pub fn deserialize<'de, D>(deserializer: D) -> Result<Duration, D::Error>
+where
+    D: Deserializer<'de>,
+{
+    UInt::deserialize(deserializer).map(|ms| Duration::from_millis(ms.into()))
+}
+
+#[cfg(test)]
+mod tests {
+    use std::time::Duration;
+
+    use serde::{Deserialize, Serialize};
+    use serde_json::json;
+
+    #[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
+    struct DurationTest {
+        #[serde(with = "super")]
+        timeout: Duration,
+    }
+
+    #[test]
+    fn deserialize() {
+        let json = json!({ "timeout": 3000 });
+
+        assert_eq!(
+            serde_json::from_value::<DurationTest>(json).unwrap(),
+            DurationTest { timeout: Duration::from_secs(3) },
+        );
+    }
+
+    #[test]
+    fn serialize() {
+        let test = DurationTest { timeout: Duration::from_millis(7000) };
+        assert_eq!(serde_json::to_value(test).unwrap(), json!({ "timeout": 7000 }));
+    }
+}