Set Away to the time that the away-for time started

I have an automation that sets a state to not_home after someone has been away for 8 hours.

But how can I configure / override so that the away time reflects the start of the 8 hours and not the end of it.

Lets say someone leaves at 12:00pm and trigger is set to “not_home” after 8 hours, once those 8 hours have elapsed, I want to set the away time to 12pm and not 8pm ?

Subtract 8 hours from now() and store that.

"{{ now() - timedelta(hours = 8) }}"