(a) Unless an agency establishes a minimum charge of less than one hour, or establishes a different minimum charge through negotiations, the minimum charge for leave is one hour, and additional charges are in multiples thereof. If an employee is unavoidably or necessarily absent for less than one hour, or tardy, the agency, for adequate reason, may excuse him without charge to leave.

(b) When an employee is charged with leave for an unauthorized absence or tardiness, the agency may not require him to perform work for any part of the leave period charged against his account.