Boards may not be started if there is less than 3 minutes left on the clock. For this purpose a board is deemed to have started once play has finished on the previous board and at least one player has taken his cards from the next board.
It is the responsibility of all players at the table to enforce this rule. It is no excuse to say that players were unaware of the time.
The director should be called and he or she will award an adjusted score (usually 50%/50%).
On the penultimate round, the director will often allow late play of the last board but he or she must be called and permission obtained. Play will not be allowed if the move has been called or is close to being called. There is no automatic time limit on the last round.
If players start a board after the 3 minute mark, the director may terminate play and may also impose a procedural penalty on both pairs.