diff --git a/src/core/pdu/mod.rs b/src/core/pdu/mod.rs
index 7cc649c8..32ec19c6 100644
--- a/src/core/pdu/mod.rs
+++ b/src/core/pdu/mod.rs
@@ -119,7 +119,7 @@ impl PduEvent {
 		// deliberately allowing for the possibility of negative age
 		let now: i128 = MilliSecondsSinceUnixEpoch::now().get().into();
 		let then: i128 = self.origin_server_ts.into();
-		let this_age: i128 = now - then;
+		let this_age: i128 = now.saturating_sub(then);
 
 		unsigned.insert("age".to_owned(), to_raw_value(&this_age).unwrap());
 		self.unsigned = Some(to_raw_value(&unsigned).expect("unsigned is valid"));