diff --git a/custom_components/planta/pyplanta/__init__.py b/custom_components/planta/pyplanta/__init__.py
index 120d15b..53084ce 100644
--- a/custom_components/planta/pyplanta/__init__.py
+++ b/custom_components/planta/pyplanta/__init__.py
@@ -103,7 +103,8 @@ async def _refresh_token(self) -> None:
         async with self._lock:
             if self._is_token_valid():
                 return
-            self._client.headers.pop("Authorization")
+            if "Authorization" in self._client.headers:
+                self._client.headers.pop("Authorization")
             resp = await self._request(
                 "POST",
                 "https://securetoken.googleapis.com/v1/token",
diff --git a/custom_components/planta/sensor.py b/custom_components/planta/sensor.py
index 670f469..1d21f32 100644
--- a/custom_components/planta/sensor.py
+++ b/custom_components/planta/sensor.py
@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
-from datetime import datetime
+from datetime import datetime, timedelta, timezone
 import logging
 
 from homeassistant.components.sensor import (
@@ -12,12 +12,13 @@
     SensorEntityDescription,
     SensorStateClass,
 )
-from homeassistant.const import EntityCategory, UnitOfLength
+from homeassistant.const import EntityCategory, UnitOfLength, UnitOfTime
 from homeassistant.core import HomeAssistant, callback
 from homeassistant.helpers.entity_platform import AddEntitiesCallback
+from homeassistant.helpers.event import async_track_time_interval
 
 from . import PlantaConfigEntry
-from .coordinator import PlantaCoordinator
+from .coordinator import PlantaCoordinator, PlantaPlantCoordinator
 from .entity import PlantaEntity
 
 _LOGGER = logging.getLogger(__name__)
@@ -118,6 +119,16 @@ class PlantaSensorEntityDescription(SensorEntityDescription):
         device_class=SensorDeviceClass.TIMESTAMP,
         entity_category=EntityCategory.DIAGNOSTIC,
     ),
+    PlantaSensorEntityDescription(
+        key="time_since_last_watering",
+        field="watering",
+        translation_key="time_since_last_watering",
+        device_class=SensorDeviceClass.DURATION,
+        entity_category=EntityCategory.DIAGNOSTIC,
+        native_unit_of_measurement=UnitOfTime.SECONDS,
+        suggested_unit_of_measurement=UnitOfTime.DAYS,
+        state_class=SensorStateClass.MEASUREMENT,
+    ),
 )
 
 
@@ -158,10 +169,21 @@ class PlantaHistorySensorEntity(PlantaEntity, SensorEntity):
     """Planta history sensor entity."""
 
     entity_description: PlantaSensorEntityDescription
+    coordinator: PlantaPlantCoordinator
 
     async def async_added_to_hass(self) -> None:
         self._handle_coordinator_update()
         await super().async_added_to_hass()
+        if self.entity_description.device_class == SensorDeviceClass.DURATION:
+            self.async_on_remove(
+                async_track_time_interval(
+                    self.hass, self._update_entity_state, timedelta(minutes=15)
+                )
+            )
+
+    async def _update_entity_state(self, now: datetime | None = None) -> None:
+        """Update the state of the entity."""
+        self._handle_coordinator_update()
 
     @callback
     def _handle_coordinator_update(self) -> None:
@@ -170,4 +192,8 @@ def _handle_coordinator_update(self) -> None:
             return
         value = self.coordinator.data["stats"][self.entity_description.field]["latest"]
         self._attr_native_value = datetime.fromisoformat(value) if value else None
+        if self.entity_description.device_class == SensorDeviceClass.DURATION:
+            self._attr_native_value = (
+                datetime.now(timezone.utc) - self._attr_native_value
+            ).total_seconds()
         super()._handle_coordinator_update()
diff --git a/custom_components/planta/strings.json b/custom_components/planta/strings.json
index 0db4d7e..d10d626 100755
--- a/custom_components/planta/strings.json
+++ b/custom_components/planta/strings.json
@@ -84,7 +84,8 @@
         }
       },
       "scheduled_watering": { "name": "Scheduled watering" },
-      "size": { "name": "Size" }
+      "size": { "name": "Size" },
+      "time_since_last_watering": { "name": "Time since last watering" }
     }
   }
 }
diff --git a/custom_components/planta/translations/en.json b/custom_components/planta/translations/en.json
index 448cabd..97f335c 100755
--- a/custom_components/planta/translations/en.json
+++ b/custom_components/planta/translations/en.json
@@ -84,7 +84,8 @@
         }
       },
       "scheduled_watering": { "name": "Scheduled watering" },
-      "size": { "name": "Size" }
+      "size": { "name": "Size" },
+      "time_since_last_watering": { "name": "Time since last watering" }
     }
   }
 }