In computing any period of time prescribed by or allowed by this subchapter, except in agreement bar situations described in Sec. 1422.3(c) of this subchapter, the day of the act, event, or default from or after which the designated period of time begins to run, shall not be included. The last day of the period so computed is to be included unless it is a Saturday, Sunday, or a Federal legal holiday in which event the period shall run until the end of the next day which is neither a Saturday, Sunday, or a Federal legal holiday: Provided, however, In agreement bar situations described in Sec. 1422.3 (c) and (d), if the sixtieth (60th) day prior to the expiration date of an agreement falls on Saturday, Sunday or a Federal legal holiday, a petition, to be timely, must be received by the close of business of the last official workday preceding the sixtieth (60th) day. When the period of time prescribed or allowed is seven (7) days or less, intermediate Saturdays, Sundays, and Federal legal holidays shall be excluded from the computations. When this subchapter requires the filing of any paper, such document must be received by the Board or the officer or agent designated to receive such matter before the close of business on the last day of the time limit, if any, for such filing or extension of time that may have been granted.