Most of the time, it is 100 hours from your departure city’s time zone. If you are upgrading SLC/DEN/BOS, then 100 hours from SLC local departure time. If you are upgrading only DEN/BOS of your SLC/DEN/BOS itinerary, then I presume it will be DEN’s local departure time. I am not 100% sure. Maybe someone else has actual experience can share.

I have read in other threads that FTer was upgraded outside of his/her upgrade window period.