The determination of one year's service credit shall be governed as follows.
1. All service in one calendar year. The board may not allow more than one year's service credit for all the service occurring in one calendar year.
2. Absence without pay. The board may not allow service credit for a period of absence without pay of more than a month's duration for a full-time position.
3. Board determination. The board shall determine by appropriate rules how much service in any year qualifies for one year's service credit. Service rendered for the full normal working time in any year shall be equivalent to one year's service credit.