Subject to subsections (b) and (c) of this section, the Board of Trustees shall adopt regulations that specify the period of employment in a year that is equal to 1 year of service credit. The Board of Trustees may not allow more than 1 year of service credit for a calendar year. A member is entitled to receive 1 month of service credit for employment for a month or a part of a month if a member contribution is made in that month.