How to combine two input_datetime helpers and use the result in a device_class timestamp sensor to trigger time based automations

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Hello HA community,

so far I’ve always been able to solve my issues by reading the docs and searching on the forum but now I’m stuck and need some advise.

I want to create some automations for my garden watering. I have 5 different water circuits in my garden. The watering should start at a specific time and each circuit should water the plants separately (so only 1 circuit runs at a time). All water circuits should run consecutively.

I have created two input_datetime helpers. The first one defines the start date and the second input should define the duration of the watering. The second circuit should start 15:15h + 6 Minutes.

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Start Time #2 is a sensor with the device_class timestamp (I read that it needs to be a device class timestamp sensor in order to use it as a time trigger for automations).

  - sensor:
      - name: "Start Time #2"
        state: "{{ ((state_attr('input_datetime.test', 'timestamp') | int + state_attr('input_datetime.test_2', 'timestamp') | int) | timestamp_custom('%H:%M:%S', false)) }}"
        device_class: timestamp

The template gives me the correct time (15:21:00) but it’s not in the right format:

Log: sensor.start_time_2 rendered invalid timestamp: 15:21:00

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Any help or suggestions how I can achieve that would be great!

Thank you.