diff --git a/shiftregister/app/sipgate/history.py b/shiftregister/app/sipgate/history.py
index c2d050f..6f79c35 100644
--- a/shiftregister/app/sipgate/history.py
+++ b/shiftregister/app/sipgate/history.py
@@ -30,7 +30,9 @@ def list_incoming_sms(from_dt=None):
 
     if from_dt is not None:
         params["from"] = (
-            from_dt.astimezone(timezone.utc).isoformat(timespec="seconds") + "Z"
+            from_dt.astimezone(timezone.utc)
+            .isoformat(timespec="seconds")
+            .replace("+00:00", "Z")
         )
 
     items = []