diff --git a/artiq/master/worker_impl.py b/artiq/master/worker_impl.py
index 7df76e5f0..7e8ea7ff5 100644
--- a/artiq/master/worker_impl.py
+++ b/artiq/master/worker_impl.py
@@ -234,7 +234,7 @@ def main():
                     dataset_mgr.write_hdf5(f.create_group("datasets"))
                     f["artiq_version"] = artiq_version
                     f["rid"] = rid
-                    f["start_time"] = start_time
+                    f["start_time"] = int(time.mktime(start_time))
                     f["expid"] = pyon.encode(expid)
                 put_object({"action": "completed"})
             elif action == "examine":