diff --git a/changelog.d/10355.bugfix b/changelog.d/10355.bugfix
new file mode 100644
index 0000000000..92df612011
--- /dev/null
+++ b/changelog.d/10355.bugfix
@@ -0,0 +1 @@
+Fix newly added `synapse_federation_server_oldest_inbound_pdu_in_staging` prometheus metric to measure age rather than timestamp.
diff --git a/synapse/storage/databases/main/event_federation.py b/synapse/storage/databases/main/event_federation.py
index c4474df975..4e06938849 100644
--- a/synapse/storage/databases/main/event_federation.py
+++ b/synapse/storage/databases/main/event_federation.py
@@ -1230,7 +1230,9 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore, SQLBas
                 "SELECT coalesce(min(received_ts), 0) FROM federation_inbound_events_staging"
             )
 
-            (age,) = txn.fetchone()
+            (received_ts,) = txn.fetchone()
+
+            age = self._clock.time_msec() - received_ts
 
             return count, age