“Base days” means the number of days of creditable service the employer requires the member’s class of employees to perform in a school year during the member’s most recent year of creditable service to earn one year of service credit. Base days shall not include school and legal holidays and shall not be less than the minimum standard specified in Section 22138.5. For those standards identified in Section 22138.5 that are expressed only in terms of hours, the number of hours shall be divided by six to determine the number of base days. The number of base days shall not be less than 175.