Calculations with times

I want my heating started depending on the outside temperature. I have a time (F.I. 8:00) when the heating has to be on temperature. I know the time it takes to heat the house at different outside temperatures. How do I calculate the starting time of the heating?

"{{ today_at(08:00) - timedelta(hours=<hours to heat here,minutes=<minute_to_heat_here>>) }}"