diff --git a/server.py b/server.py
index 569ca8e..421f4bc 100644
--- a/server.py
+++ b/server.py
@@ -43,5 +43,5 @@ def update(bike_id):
     
     bike.update(lat, lon, speed, bearing)
 
-    return jsonify(last_update=bike.last_update,
+    return jsonify(last_update=bike.last_update.isoformat(),
                    **bike.flock_data())