No one but the ref really knows what time it is: In practice the only official clock is usually the stopwatch in the referee's hand or a digital watch on their wrist (called "keeping time on the field"), and there is no particular requirement that the referee inform teams how much time remains. Even if a publicly visible scoreboard clock is available, this at best only tracks official time elapsed, and not official time remaining, because of the discretionary power of the referee to add time to compensate for time lost through delays.

Game's only over when the ref says it is: Remarkably, the rules do not require the referee to inform anyone, not even the assistant referees, how much, if any time s/he intends to add to the end of the game, and no one but the referee knows when time will run out and s/he will end of the game with the distinctive triple signature of the whistle, tweet-tweet-tweeeeet!