Add documentation for supporting offsets (#33611)

source/_docs/automation/trigger.markdown

@@ -751,9 +751,32 @@ automation:
           entity_id: light.bedroom
 ```
 
+### Sensors of datetime device class with offsets
+
+When the time is provided using a sensor of the timestamp device class, an offset can be provided. This offset will be added to (or subtracted from when negative) the sensor value.
+
+For example, this trigger fires 5 minutes before the phone alarm goes off.
+
+```yaml
+automation:
+  - trigger:
+      - platform: time
+        at:
+          entity_id: sensor.phone_next_alarm
+          offset: -00:05:00
+    action:
+      - service: light.turn_on
+        target:
+          entity_id: light.bedroom
+```
+
+{% important %}
+When using a positive offset the trigger might never fire. This is due to the sensor changing before the offset is reached. For example, when using a phone alarm as a trigger, the sensor value will change to the new alarm time when the alarm goes off, which means this trigger will change to the new time as well.
+{% endimportant %}
+
 ### Multiple times
 
-Multiple times can be provided in a list. Both formats can be intermixed.
+Multiple times can be provided in a list. All formats can be intermixed.
 
 ```yaml
 automation:
@@ -762,6 +785,8 @@ automation:
       at:
         - input_datetime.leave_for_work
         - "18:30:00"
+        - entity_id: sensor.bus_arrival
+          offset: "-00:10:00"
 ```
 
 ## Time pattern trigger