diff --git a/rust/src/duration.rs b/rust/src/duration.rs
index 7eb866743a..6c2e2653d1 100644
--- a/rust/src/duration.rs
+++ b/rust/src/duration.rs
@@ -31,26 +31,22 @@ fn duration_module(py: Python<'_>) -> PyResult<&Bound<'_, PyAny>> {
 /// Mirrors the `synapse.util.duration.Duration` Python class.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct SynapseDuration {
-    microseconds: u64,
+    milliseconds: u64,
 }
 
 impl SynapseDuration {
-    /// For now we only need to create durations from milliseconds.
+    /// Creates a `SynapseDuration` from a number of milliseconds.
     pub const fn from_milliseconds(milliseconds: u64) -> Self {
-        Self {
-            // We saturate at u64::MAX microseconds to avoid overflow, which
-            // means that the maximum duration we can represent is approximately
-            // 584,942 years.
-            microseconds: milliseconds.saturating_mul(1_000),
-        }
+        Self { milliseconds }
     }
 
-    pub const fn from_hours(hours: u64) -> Self {
+    /// Creates a `SynapseDuration` from a number of hours.
+    pub const fn from_hours(hours: u32) -> Self {
+        // We take a u32 here so that we know the multiplication won't overflow.
+        // We could instead panic, but that is unstable in a const context (for
+        // the current MSRV 1.82).
         Self {
-            // We saturate at u64::MAX microseconds to avoid overflow, which
-            // means that the maximum duration we can represent is approximately
-            // 584,942 years.
-            microseconds: hours.saturating_mul(3_600_000_000),
+            milliseconds: (hours as u64) * 3_600_000,
         }
     }
 }
@@ -62,7 +58,7 @@ impl<'py> IntoPyObject<'py> for SynapseDuration {
 
     fn into_pyobject(self, py: Python<'py>) -> Result<Self::Output, Self::Error> {
         let duration_module = duration_module(py)?;
-        let kwargs = [("microseconds", self.microseconds)].into_py_dict(py)?;
+        let kwargs = [("milliseconds", self.milliseconds)].into_py_dict(py)?;
         let duration_instance = duration_module.call_method("Duration", (), Some(&kwargs))?;
         Ok(duration_instance.into_bound())
     }
@@ -75,7 +71,7 @@ impl<'py> IntoPyObject<'py> for &SynapseDuration {
 
     fn into_pyobject(self, py: Python<'py>) -> Result<Self::Output, Self::Error> {
         let duration_module = duration_module(py)?;
-        let kwargs = [("microseconds", self.microseconds)].into_py_dict(py)?;
+        let kwargs = [("milliseconds", self.milliseconds)].into_py_dict(py)?;
         let duration_instance = duration_module.call_method("Duration", (), Some(&kwargs))?;
         Ok(duration_instance.into_bound())
     }