In a recently played MLS game, the referee did not follow the standardized procedures by which additional time is allowed in a period of play. This problem also occurred in recent UEFA matches. The UEFA Referee Committee determined that the referees were ending the game before the full added time was played. They have issued a directive to their referees reminding them to allow the full time allotted for each period of play.
USSF has issued a position paper “Allowance for Time Lost”, April 22, 208, which can be found at http://www.ussoccer.com/laws/papers.jsp.html
Allowance is made in any period of play (including extra time) for time lost through substitutions, assessment of injuries, removal of players from the field of play for treatment, time wasting and any other cause. Among examples of "other cause" would be the need for the officiating team to confer regarding the identity of players committing misconduct; confrontations with officials; the removal of streamers, debris or other objects thrown on to the field which interrupts the game; pitch incursions by spectator(s); and so forth.
This minimum time does not indicate the exact amount of time left in the match nor does it preclude more time being added to the allowance for any subsequent injuries or additional delays (including time wasting). The specific amount of the allowance is at the discretion of the referee. During the two minutes before the expiration of each period of play the Referee must inform the Fourth Official, either visually or verbally, of the amount of time allowed. That information is conveyed to both teams.
It is important to note that once the time has been announced, either by the fourth official or on the electronic sign board, the time cannot be reduced.