NFL spokesman Michael Signora delivered an explanation from the league to NFL Media Insider Ian Rapoport. The statement, in full:
"With 2:56 remaining in the fourth quarter, San Diego kicked off to Pittsburgh. The kick resulted in a touchback. By rule, the game clock does not start if the receiving team possesses the ball in the end zone and does not carry it into the field of play.
"Because of an error by the clock operator, the game clock was incorrectly started before the Steelers' first play from scrimmage following the touchback. That first down snap came with 2:38 left to play instead of 2:56, a difference of 18 seconds.
"The official game time is kept on the stadium scoreboard, but it is the responsibility of the side judge to supervise the timing of the game. Had the side judge or any of the other six on-field officials noticed the timing error, they could have corrected it.
"The game clock is not subject to instant replay review unless there is a timing issue on the last play of the first half or the last play of the game. The performance of the clock operator and game officials will be reviewed per the standard procedure for reviewing every play of every game."
NFL VP of officiating Dean Blandino told NFL Total Access on Tuesday night reiterated that this was a mistake by the clock operator, but added that the officials on the field "should be checking the clock to make sure that we don't lose time."
"Even though we can't get involved from New York with clock situations," Blandino added, "we're going to look at it and review our process to make sure that we have the right controls in place so it doesn't happen again."
NFL clock operators are hired by the league and usually have college football officiating experience. Clock operators typically live in close geographic proximity to the stadium where they are working, though the league uses operators from outside the region of the home team's stadium during the postseason.