UHC catagory "Stronghold" I'd suggest for the rules the time starts first frame you can see your teammates pop up in the chat and the timer ends when you fully broke a block from the stronghold (as seen in my screenshots that I provide down below) this way its the most easiest to determen a time

First frame of teammates: https://prnt.sc/tndagg

First frame of having the block fully broken: https://prnt.sc/tndb2j