Except as otherwise provided in the regulations of this Subchapter, computation of time is based upon the following:

1. (1) Except as otherwise provided, computation of time is based upon calendar days.
2. (2) In computing any period of prescribed time, the day on which the designated period of time begins is not included. The last day of the period is included unless it is a Saturday, Sunday, or legal holiday on which the Department is not open for business, in which event the period runs until the end of the next day which is not a Saturday, Sunday, or legal holiday.
3. (3) Intermediate Saturdays, Sundays, or legal holidays are excluded from the computation when the period or prescribed time is seven days or less.