RSA 80:55 Timely Mailing.

I.
General Rule. Any report, claim, tax return, statement and other document, relative to tax matters, required or authorized to be filed with or any payment made to the state or to any political subdivision thereof which is:
(a) Transmitted through the United States mail, shall be deemed filed and received by the state or political subdivision on the date shown by the post office cancellation mark stamped upon the envelope or other appropriate wrapper containing it;
(b) Mailed but not received by the state or political subdivision or where received and the cancellation mark is illegible, erroneous, or omitted, shall be deemed filed and received on the date it was mailed if the sender establishes by competent evidence that the report, claim, tax return, statement,