The players are under contractual regs and rules the moment they 'show up to work'.  They get paid to practice and all of the pre game stuff, and can be fined for being late, not practicing, as well a zillion other infractions.  It doesn't just apply while the game clock is ticking.  So yes, the players are on the clock long before the Anthem.

Do you think if they show up half drunk, but pass a breathalizer right before kickoff, that the coach will be cool with that?  ;)