Rule 2 – Section 29 – Article 2 – Play Clock
a. Each stadium should have a visual play clock at each end of the playing enclosure. The play clock (if provided) must be capable of counting down from both 40 seconds and 25 seconds. It should automatically default to 40 seconds and start immediately upon being reset by the play-clock operator when any official signals that the ball is dead after a play.
b. Otherwise, the play clock is any device under the direction of the appropriate official used to time the 40/25 seconds between end of the previous play or the ready for play signal and the ball being put in play.