diff --git a/little_boxes/activitypub.py b/little_boxes/activitypub.py
index 35995b1..dafc27f 100644
--- a/little_boxes/activitypub.py
+++ b/little_boxes/activitypub.py
@@ -73,7 +73,7 @@ def format_datetime(dt: datetime) -> str:
     if dt.tzinfo is None:
         raise ValueError("datetime must be tz aware")
 
-    return dt.astimezone(timezone.utc).replace(microsecond=0).isoformat() + "Z"
+    return dt.astimezone(timezone.utc).replace(microsecond=0).isoformat().replace("+00:00", "Z")
 
 
 class ActivityType(Enum):