The standard work shift for employees occupying full-time positions in this unit consists of 40 hours in a 7-day period unless otherwise specified by the Board of Supervisors. For employees working twelve-hour shifts the standard work period consists of 168 hours in a 28-day period. The appointing authority shall fix the hours of work with due regard for the convenience of the public and the laws of the State and County. Employees occupying part-time positions shall work such hours and schedules as the Board and the appointing authority shall prescribe.