diff --git a/crates/web5/src/rfc3339.rs b/crates/web5/src/rfc3339.rs
index 9025ffad..1e6e46f9 100644
--- a/crates/web5/src/rfc3339.rs
+++ b/crates/web5/src/rfc3339.rs
@@ -10,7 +10,7 @@ where
     S: Serializer,
 {
     let datetime: chrono::DateTime<Utc> = (*time).into();
-    let s = datetime.to_rfc3339();
+    let s = datetime.to_rfc3339_opts(chrono::SecondsFormat::Secs, true);
     serializer.serialize_str(&s)
 }