Q. What do you do if the field is not set up and ready for play when the game is ready to start?
A. If we go "by the book" then if either the teams or the field are not ready at game time, the match should be abandoned and the referee report the facts in the game report. If a league allows a grace period then it would be game time + 10/15 minutes. This is very important for competitive matches. Our travel leagues have upheld protests against referee actions who extended the amount of time to start by even five minutes. Start time means start time.
For our recreational games the intelligent ref should normally permit the match to start late but inform both coaches the halves will be reduced by x minutes because they were not ready to play on time.
If it is the first match of the day, as field setup will be, and the ref abandoned the match, they do have to stay around for their next match. If it is their last match of the day, as soon as they abandon the match they are free to leave.