Mont. Code Ann. § 15-70-103 — Time Of Mailing And Filing

15-70-103 . Time of mailing and filing. (1) Any claim, statement, remittance, or other document which is transmitted to this state through the United States mail shall be deemed filed and received by this state on the date shown by the post-office cancellation mark stamped upon the envelope or other appropriate wrapper containing it. Any claim, statement, remittance, or other document which is mailed but not received by this state or where received with a cancellation mark that is illegible, erroneous, or omitted shall be deemed filed and received on the date mailed if the sender establishes by competent evidence that the claim, statement, remittance, or other document was deposited in the United States mail on or before the date due for filing. In cases of such nonreceipt of a claim, statement, remittance, or other document, the sender must file with the state a duplicate within 30 days after written notification is given to the sender by the state of its nonreceipt of such claim, statement, remittance, or other document.
(2) If any claim, statement, remittance, or other document is sent by United States registered mail, certified mail, or certificate of mailing, a record authenticated by the United States post office of such registration, certification, or certificate shall be considered competent evidence that the report, claim, tax return, statement, remittance, or other document was mailed to the addressee, and the date of registration, certification, or certificate shall be deemed the postmarked date.
(3) If the date for filing any claim, statement, remittance, or other document falls upon a Saturday, Sunday, or legal holiday, the filing shall be considered timely if done on the next business day. Such reports shall be considered filed or received on the date or as provided in this chapter.