How many hours since automation last ran

Configuration
7

The result of that subtraction is a timedelta object.

A timedelta object has a total_seconds() method that reports its value in seconds. Divide it by 3600 to get hours (as a floating point value).

{{ (now() - state_attr('automation.write_hot_tub_log','last_triggered')).total_seconds() / 3600 }}

I feel the result is to be expected given that you just manually triggered the automation.

last_triggered represents the time when the automation started executing its action (it has already successfully processed its trigger and action).

If the automation references its last_triggered value in its trigger or condition it will be, what you expect, namely the last time it triggered (i.e. not just a moment ago but some time in the past). However, upon executing its action the value of last_triggered is updated to the latest trigger time (i.e merely a moment ago).

If you want something to report the elapsed hours since the automation executed, you can use a Template Sensor.

template:
  - sensor:
      - name: Hot Tub Elapsed Hours
        state: "{{ (now() - state_attr('automation.write_hot_tub_log','last_triggered')).total_seconds() / 3600 }}"

However, if your goal is to have an action in automation.write_hot_tub_log reference its own previous value of last_triggered then you can’t use a Template Sensor (because it will get updated the moment the automation’s last_triggered is updated).

EDIT

FWIW, this might be of interest to you:

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