diff --git a/kasa/smartdevice.py b/kasa/smartdevice.py
index fabf26b32..30468c45f 100755
--- a/kasa/smartdevice.py
+++ b/kasa/smartdevice.py
@@ -598,7 +598,7 @@ def on_since(self) -> Optional[datetime]:
 
         on_time = self.sys_info["on_time"]
 
-        return datetime.now() - timedelta(seconds=on_time)
+        return datetime.now().replace(microsecond=0) - timedelta(seconds=on_time)
 
     @property  # type: ignore
     @requires_update