The warrant for any town meeting shall be under the hands of the selectmen, and shall prescribe the place, day and hour of the meeting, and, if there is an election at said meeting, in which an official printed ballot containing more than one name is used, the warrant therefor shall prescribe the time the polls are to open and also an hour before which the polls may not close. A town meeting may vote to keep the polls open to a later hour but may not vote to close the polls at an earlier hour than that prescribed by the selectmen hereunder. The subject matter of all business to be acted upon at the town meeting shall be distinctly stated in the warrant, and nothing done at any meeting, except the election of any town officer required by law to be made at such meeting, shall be valid unless the subject thereof is so stated. Provided that in a case where the article in the warrant calls for the appropriation of a specific sum of money, the sum of money appropriated thereunder may be decreased or increased by the vote of the town, provided further that in a town under the municipal budget act no increase shall be valid which would violate the provisions of N.H. Rev. Stat. § 32:18, except as provided in N.H. Rev. Stat. § 32:18-a.