The day of the act, event or default after which the designated period of time begins to run is not to be included in computing any period of time prescribed or allowed by the personnel appeals board rules. The last day of the period so computed is to be included unless it is a Saturday, Sunday or a legal holiday, in which event the period runs until the end of the next day which is neither a Saturday, Sunday nor a holiday. When the period of time prescribed or allowed is less than seven days, intermediate Saturdays, Sundays and holidays shall be excluded in the computation.