N.H. Rev. Stat. Ann. § 168-A:3-a

If the child is 3 months old or older, the father's liability for past education and support under RSA 168-A:1 is limited to amounts accrued from the date of service of the petition on the father or, if the court finds that the father is willfully avoiding service, from such date as the court determines is just and equitable. There is no limitation on retroactive support if the child is less than 3 months old.