Trying to use template during service: calendar.create_event

Hi guys, so far I have been successful in finding solutions to my problems thanks to all you guys posting so nicely. But this time I can’t seem to solve it.
I aim to create calendar items in a dedicated calendar called “rainfall” after each rainfall event, so I have a nice way of going back to see from when to when how much rainfall happened.

my call would look like this:

service: calendar.create_event
data:
  summary: "{{ states(sensor.garden_rain_per_ocurrence)}}mm rain
  start_date_time: "{{ states(input_datetime.garden_rain_start) | as_datetime }}"
  end_date_time: "{{ states(input_datetime.garden_rain_end) | as_datetime }}"
target:
  entity_id: calendar.rainfall

error message in the logs as per below:

Logger: homeassistant.components.websocket_api.http.connection
Source: components/websocket_api/connection.py:231
Integration: Home Assistant WebSocket API (documentation, issues)
First occurred: 12:01:15 AM (1 occurrences)
Last logged: 12:01:15 AM

[546794216016] Error handling message: Error rendering data template: UndefinedError: 'input_datetime' is undefined (unknown_error) xxx from xxx.xxx.xxx.xxx (Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/114.0)

what am I overlooking?

Thanks,
Holger