Mich. Comp. Laws § 38.1376

Sec. 76. On and after the effective date of this section, a member who is otherwise entitled to purchase service credit for active duty in the armed forces under section 73 or 74, or for maternity or paternity or child rearing under section 75, 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 73, 74, and 75, but computation of the amount of payment due shall be made separately for each purchase.