N.H. Rev. Stat. Ann. § 281-A:16

For the purpose of determining the date of injury for an occupational disease, the date of injury shall be taken to be the last date of injurious exposure to the hazards of such disease or the date on which the employee first knew or reasonably should have known of the condition and its relationship to the employee's employment, whichever is the later. For an injury caused by cumulative trauma, the date of injury shall be the date of first medical treatment. For an injury or condition aggravated by cumulative trauma, the date of injury shall be the date of first medical treatment for the aggravation.