Miss. Code § 25-13-5
Creditable service on which a member's service or disability retirement benefit is based shall consist of "prior service" and membership service. Prior service shall mean service performed for the Highway Safety Patrol as defined in Section 25-13-3 before July 1, 1958 and service performed as a sworn agent for the Mississippi Bureau of Narcotics prior to March 29, 1991. No prior service credits shall be granted any person who re-enters the employment of the Highway Safety Patrol after July 1, 1958, except that any former sworn officer of the Highway Safety Patrol who returns to the Highway Safety Patrol in any capacity, and who has had not less than two (2) years of prior service as a sworn officer of the Highway Safety Patrol, and who was disabled by wounds or accident in line of duty, may become a member of the Highway Safety Patrol Retirement System with full credit for any previous service as set forth in Section 25-13-3 with the Highway Safety Patrol. Membership service shall mean all services for which credit may be allowed under this chapter subsequent to July 1, 1958, and all lawfully credited unused leave as of the date of withdrawal from service, as certified by the appointing authority.
Miss. Code § 25-13-5