diff --git a/homeassistant/components/sensor/__init__.py b/homeassistant/components/sensor/__init__.py
index f3b9a24a15d..75fea9e044b 100644
--- a/homeassistant/components/sensor/__init__.py
+++ b/homeassistant/components/sensor/__init__.py
@@ -119,6 +119,6 @@ class SensorEntity(Entity):
     def state_attributes(self) -> dict[str, Any] | None:
         """Return state attributes."""
         if last_reset := self.last_reset:
-            return {ATTR_LAST_RESET: last_reset}
+            return {ATTR_LAST_RESET: last_reset.isoformat()}
 
         return None
diff --git a/tests/components/utility_meter/test_sensor.py b/tests/components/utility_meter/test_sensor.py
index 9157ba738c7..54854ac9668 100644
--- a/tests/components/utility_meter/test_sensor.py
+++ b/tests/components/utility_meter/test_sensor.py
@@ -200,12 +200,12 @@ async def test_restore_state(hass):
     state = hass.states.get("sensor.energy_bill_onpeak")
     assert state.state == "3"
     assert state.attributes.get("status") == PAUSED
-    assert state.attributes.get("last_reset") == dt_util.parse_datetime(last_reset)
+    assert state.attributes.get("last_reset") == last_reset
 
     state = hass.states.get("sensor.energy_bill_offpeak")
     assert state.state == "6"
     assert state.attributes.get("status") == COLLECTING
-    assert state.attributes.get("last_reset") == dt_util.parse_datetime(last_reset)
+    assert state.attributes.get("last_reset") == last_reset
 
     # utility_meter is loaded, now set sensors according to utility_meter:
     hass.bus.async_fire(EVENT_HOMEASSISTANT_START)
@@ -348,12 +348,13 @@ async def _test_self_reset(hass, config, start_time, expect_reset=True):
     state = hass.states.get("sensor.energy_bill")
     if expect_reset:
         assert state.attributes.get("last_period") == "2"
-        assert state.attributes.get("last_reset") == now
+        assert state.attributes.get("last_reset") == now.isoformat()
         assert state.state == "3"
     else:
         assert state.attributes.get("last_period") == 0
         assert state.state == "5"
-        assert state.attributes.get("last_reset") == dt_util.parse_datetime(start_time)
+        start_time_str = dt_util.parse_datetime(start_time).isoformat()
+        assert state.attributes.get("last_reset") == start_time_str
 
 
 async def test_self_reset_quarter_hourly(hass, legacy_patchable_time):