29 C.F.R. § 778.319
Paying for but not counting hours worked
In some contracts provision is made for payment for certain hours, which constitute working time under the Act, coupled with a provision that these hours will not be counted as working time. Such a provision is a nullity. If the hours in question are hours worked, they must be counted as such in determining whether more than the applicable maximum hours have been worked in the workweek. If more hours have been worked, the employee must be paid overtime compensation at not less than one and one-half times his regular rate for all overtime hours. A provision that certain hours will be compensated only at straight time rates is likewise invalid. If the hours are actually hours worked in excess of the applicable maximum hours standard, extra half-time compensation will be due regardless of any agreement to the contrary.