Time trigger can also accept an input_datetime Entity ID (#14207)

Co-authored-by: Franck Nijhof <git@frenck.dev>

source/_docs/automation/trigger.markdown

@@ -282,9 +282,10 @@ automation:
 The `for` template(s) will be evaluated when the `value_template` becomes `true`.
 
 <div class='note warning'>
+  
 Rendering templates with time (`now()`) is dangerous as trigger templates are only updated based on entity state changes.
-</div>
 
+</div>
 
 As an alternative, providing you include the sensor [time](/integrations/time_date/) in your configuration, you can use the following template:
 
@@ -303,7 +304,11 @@ which will evaluate to `True` if `YOUR.ENTITY` changed more than 300 seconds ago
 
 ### Time trigger
 
-The time trigger is configured to fire once at a specific point in time each day.
+The time trigger is configured to fire once a day at a specific time, or at a specific time on a specific date. There are two allowed formats:
+
+#### Time String
+
+A string that represents a time to fire on each day. Can be specified as `HH:MM` or `HH:MM:SS`. If the seconds are not specified, `:00` will be used.
 
 ```yaml
 automation:
@@ -313,15 +318,55 @@ automation:
     at: "15:32:00"
 ```
 
-Or at multiple specific times:
+#### Input Datetime
+
+The Entity ID of an [Input Datetime](/integrations/input_datetime/).
+
+has_date | has_time | Description
+-|-|-
+`true` | `true` | Will fire at specified date & time.
+`true` | `false` | Will fire at midnight on specified date.
+`false` | `true` | Will fire once a day at specified time.
+
+{% raw %}
+
+```yaml
+automation:
+  - trigger:
+      platform: state
+      entity_id: binary_sensor.motion
+      to: 'on'
+    action:
+      - service: climate.turn_on
+        entity_id: climate.office
+      - service: input_datetime.set_datetime
+        entity_id: input_datetime.turn_off_ac
+        data_template:
+          datetime: >
+            {{ (now().timestamp() + 2*60*60)
+               | timestamp_custom('%Y-%m-%d %H:%M:%S') }}
+
+  - trigger:
+      platform: time
+      at: input_datetime.turn_off_ac
+    action:
+      service: climate.turn_off
+      entity_id: climate.office
+```
+
+{% endraw %}
+
+#### Multiple Times
+
+Multiple times can be provided in a list. Both formats can be intermixed.
 
 ```yaml
 automation:
   trigger:
     platform: time
     at:
-      - "15:32:00"
-      - "20:30:00"
+      - input_datetime.leave_for_work
+      - "18:30:00"
 ```
 
 ### Time pattern trigger