Play stops on or about this official time-out near the end of each half in an NFL game
Time between plays will be 40 seconds from the end of a given play until the 
... half, the game clock will be restarted following a kickoff return, a player going 
out... [DOC] [TLE] Rule 4 Game Timing - NFL.com [PAR] Article 4 The stadium electric clock shall be the official time.