In computing any period of time prescribed or allowed by commission rules, by commission order, or by statute, the day of the act, event, or default after which the designated period of time begins to run shall not be included. The last day of the period so computed shall 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 legal holiday. When the period of time prescribed or allowed is less than seven days, intermediate Saturdays, Sundays and holidays shall not be counted.