N.H. Rev. Stat. § 21:36
The standard time within the state, except as hereinafter provided, shall be based on the mean astronomical time of the seventy-fifth degree of longitude west from Greenwich, known and designated by the federal statute as "Eastern Standard Time." At 2 o'clock ante-meridian of the second Sunday in March of each year, the standard time in this state shall be advanced one hour, at 2 o'clock ante-meridian of the first Sunday in November of each year, the standard time in this state shall, by the retarding of one hour, be made to coincide with the astronomical time hereinbefore described as Eastern Standard Time, so that between the second Sunday in March at 2 o'clock ante-meridian and the first Sunday in November at 2 o'clock ante-meridian in each year the standard time in this state shall be one hour in advance of the United States Standard Time. In all laws, statutes, orders, decrees, rules, and regulations relating to the time of performance by any officer or department of this state, or of any county, city, town, or district thereof, or relating to the time in which any rights accrue or determine, or within which any act shall or shall not be performed by any person subject to the jurisdiction of this state, or of any county, city, town, or district thereof, and in all contracts or choses in action made or to be performed in this state, it shall be understood and intended that the time shall be as set forth in this section.
RSA 21:36
1921, 15:1. 1923, 105:1, 2. PL 42 :76. 1937, 95:1. RL 51:95. 1954, 2:4. RSA 21:36. 1987, 10:1. 2011, 103:1, eff. July 30, 2011.