5 CFR §550.905

1. (a)When an employee performs duty for which a hazard pay differential is authorized, the agency must pay the hazard pay differential for the hours in a pay status on the day (a calendar day or a 24-hour period, when designated by the agency) on which the duty is performed, except as provided in paragraph (b) of this section. Hours in a pay status for work performed during a continuous period extending over 2 days must be considered to have been performed on the day on which the work began, and the allowable differential must be charged to that day.
2. (b)Employees may not be paid a hazardous duty differential for hours for which they receive annual premium pay for regularly scheduled standby duty under § 550.141, annual premium pay for administratively uncontrollable overtime work under § 550.151, or availability pay for criminal investigators under § 550.181.