Mich. Comp. Laws § 38.1635

Sec. 35. A member who is otherwise entitled to purchase service credit for active duty in the armed forces under section 32 for service in the Peace Corps or VISTA under section 33, or for maternity or paternity or child rearing under section 34, may purchase such service credit in separate increments equal to 1 or more full years, or a remaining fraction of a year, if any, or both. Partial purchase of service credit under this section shall not bar future such purchases otherwise in compliance with this section and sections 32, 33, and 34, but computation of the amount of payment due shall be made separately for each purchase.