(1) In computing any period of time for acts required by any of the rules in this chapter, the day of the act, event or default after which the designated period of time begins to run is not included. The last day of the period is the last day computed, unless it is a Saturday, Sunday or legal holiday. In that event, the period runs until the end of the next day which is not one of the aforementioned days. When the period of time prescribed or allowed is less than 11 days, intermediate Saturdays, Sundays and holidays are excluded in the computation. A half-day holiday will be considered as other days and not as a holiday.

(2) Whenever a party has a right or is required to do some act under these rules within a prescribed period after service of a notice, order or other paper upon the party, and the notice, order or other paper is served upon the party by regular mail, 3 calendar days shall be added to the prescribed period.

(3) Except for dates or prescribed times fixed by statute and not subject to modification, an ALJ, for good cause shown, may enlarge the time to perform an act if the request or motion for enlargement is made before the expiration of the prescribed period for the act. If the request or motion is made after the prescribed time has expired, enlargement may be allowed only upon a showing of excusable neglect in the failure to act. Requests or motions for enlargement of time must be made as provided in ARM 37.62.921.