Sec. 10. SERVICE CREDIT. (a) Service credit is earned in a retirement system for each month for which an employee makes the contribution required under this Act.

(b) Service credit is earned in a retirement system for each calendar year in which a volunteer answers at least 25 percent of all fire alarms determined by the board of trustees and attends at least 40 percent of all drills held by the fire department.

(c) A retirement system shall also grant service credit to a member who leaves the service of a fire department and later returns, if the break in service is attributable to service for any period as a member of the armed forces of the United States during a war or national emergency.

(d) Absence from service by an employee does not forfeit service credit accrued before the absence begins, unless membership is terminated. Absence from service by a volunteer does not forfeit service credit accrued before the absence begins.

(e) A board of trustees established under this Act may, in accordance with Section 7 of this Act, expand the circumstances under which service credit is earned.