(1) Unless specifically dictated by the Montana Code Annotated:  

(a) The time within which an act is to be done as provided in any rule or order promulgated by the commission, when expressed in days, shall be computed by excluding the first day and including the last, except that if the last day be Saturday, Sunday, or a legal holiday, the act may be done on the next succeeding regular business day.

(b) When a document is required to be filed or served on a particular day, the postmarking of the document on or before that day will satisfy this rule.