Published times. Most agencies print these in their guides or on-line.
The times posted are purely astronomical and have nothing to do with local conditions such as overcast, wooded vs. open, etc. From about Sept. 21-Dec. 21 these hours get about a minute shorter on each end of the day.
Of course if shooting light goes before the clock says so, I too would get down. My effective range shrinks as light wanes. As the last minutes of shooting light tick off my shot distance will go from 25, 20, 15. Even if the clock says I'm legal I get down (if no deer around) when my 15-yard shot disappears.
I don't get down more than a couple minutes before I can't see though. That last 30-60 minutes is often a great time for action. Most of the deer hunters I know focus on the evening hunt more than the AM hunt (but not me--75%+ of my hunts are AM). Since, in 45 years, whether with compound or recurve, my deer have gone down in less than 70 yards, usually in sight, I have no concern about fading light and losing a double-lung-shot deer.
However, if one is concerned about this, by all means follow the umpire on your shoulder, not mine.
I've downloaded the "Daylight Calculator" AP (I-Phone) from the "Bureau of Visual Affairs" (I know, who would have thunk such a bureau existed?).
In my bowhunting life, while hunting I've been officer-checked: 3 times in IN, 1 in Ontario, 1 in Wyoming. All were routine and of course no problems. The IN episode was interesting. It was in October, 1977. I had just returned home from my first job (temp) after college conducting timber inventory in Colorado. I had CO tags on my truck thus the interest from consevation law enforcement -- not too many Colorado folks hunting in Hoosierland. They checked me 3 times in 24 hours -- finally I thought to show them my NONRESIDENT Colorado elk tag. That ended the stake-out on yours truly.