N.J. Stat. § 43:16A-35
Any member of such county police department may become a member by filing with the board of trustees his election in writing to become such member on or before the date upon which this act becomes effective in such county under such rules and regulations as the board of trustees shall adopt. Each such member of a county police department so electing to become a member, shall file, immediately after becoming a member, with the board of trustees a detailed statement of all service rendered by him as policeman to his present employer prior to the date when this act becomes effective in such county, for which he claims credit. As soon as practicable thereafter the board of trustees shall verify such statement and shall issue to him a prior service certificate certifying to the aggregate length of such service allowable as creditable service but, regardless of the length of such service, no prior service certificate shall allow more than 30 years of such creditable service. Such certificate shall be final and conclusive for retirement purposes as to such service, unless thereafter modified by the board of trustees upon application made by the member within 1 year, or upon its own initiative within 1 year after the date of issuance of such prior service certificate.
N.J.S. § 43:16A-35