diff --git a/dartwrf/assim_synth_obs.py b/dartwrf/assim_synth_obs.py
index b9d295ae2e9382ec75134abefcbe24b485028f62..fdc158e2425a44d6e00ba5c8cf7168a1a0ec34d1 100755
--- a/dartwrf/assim_synth_obs.py
+++ b/dartwrf/assim_synth_obs.py
@@ -458,7 +458,7 @@ if __name__ == "__main__":
     4) Assimilate with assigned errors
     
     Args:
-        assim_time:           time of output
+        assim_time (dt.datetime):           time of output
         prior_init_time:      forecast start of prior
         prior_valid_time:     valid time of prior (may be different to assim_time)
         path to prior experiment