Trigger an automation BEFORE the time of a time helper

Configuration
3

Create a Template Trigger.

  - platform: template
    value_template: "{{ now().timestamp() | timestamp_custom('%H:%M') == (state_attr('input_datetime.whatever', 'timestamp') - 1800) | timestamp_custom('%H:%M', false) }}"

How it works:

It compares the current time to the input_datetime's time less 30 minutes (1800 seconds).

This part simply reports the current time in HH:MM format:

now().timestamp() | timestamp_custom('%H:%M')

This part takes the timestamp attribute of your input_datetime entity, subtracts 1800 seconds, and converts it to HH:MM format.

(state_attr('input_datetime.whatever', 'timestamp') - 1800) | timestamp_custom('%H:%M', false)

It compares the two results and triggers when they are equal. The template updates every minute because it contains the now() function.

NOTE

The assumption here is that the input_datetime is time-only.

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