Fix litterrobot timestamp sensor (#61208)

* Fix litterrobot timestamp sensor

* Update type

homeassistant/components/litterrobot/sensor.py

@@ -1,9 +1,11 @@
 """Support for Litter-Robot sensors."""
 from __future__ import annotations
 
+from datetime import datetime
+
 from pylitterbot.robot import Robot
 
-from homeassistant.components.sensor import SensorEntity
+from homeassistant.components.sensor import SensorEntity, StateType
 from homeassistant.config_entries import ConfigEntry
 from homeassistant.const import DEVICE_CLASS_TIMESTAMP, PERCENTAGE
 from homeassistant.core import HomeAssistant
@@ -36,7 +38,7 @@ class LitterRobotPropertySensor(LitterRobotEntity, SensorEntity):
         self.sensor_attribute = sensor_attribute
 
     @property
-    def native_value(self) -> str:
+    def native_value(self) -> StateType | datetime:
         """Return the state."""
         return getattr(self.robot, self.sensor_attribute)
 
@@ -59,10 +61,10 @@ class LitterRobotSleepTimeSensor(LitterRobotPropertySensor):
     """Litter-Robot sleep time sensor."""
 
     @property
-    def native_value(self) -> str | None:
+    def native_value(self) -> StateType | datetime:
         """Return the state."""
         if self.robot.sleep_mode_enabled:
-            return super().native_value.isoformat()
+            return super().native_value
         return None
 
     @property