RSA 400-A:32-a Timely Mailing.

I. 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 which is:
(a) Transmitted through the United States mail, shall be deemed filed and received by the state 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 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, remittance, or other document was deposited in the United States mail on or before the date due for filing; and