419. The board of appeal may, on reasonable grounds and appropriate conditions, extend a time limit or relieve one party from the consequences of his failure to comply with it, if the other party suffers no prejudice thereby.
However, the board of appeal shall not extend the time limit contemplated in section 417 nor relieve one party from the consequences of his failure to comply with it if, fifteen days before the time limit, it gave notice to the party to comply with it.