29 C.F.R. § 4.179
Contractors and subcontractors under contracts subject to the Act are required to comply with its compensation requirements throughout the period of performance on the contract and to do so with respect to all employees who in any workweek are engaged in performing work on such contracts. If such a contractor during any workweek is not exclusively engaged in performing such contracts, or if while so engaged it has employees who spend a portion but not all of their worktime in the workweek in performing work on such contracts, it is necessary for the contractor to identify accurately in its records, or by other means, those periods in each such workweek when the contractor and each such employee performed work on such contracts. In cases where contractors are not exclusively engaged in Government contract work, and there are adequate records segregating the periods in which work was performed on contracts subject to the Act from periods in which other work was performed, the compensation specified under the Act need not be paid for hours spent on non-contract work. However, in the absence of records adequately segregating non-covered work from the work performed on or in connection with the contract, all employees working in the establishment or department where such covered work is performed shall be presumed to have worked on or in connection with the contract during the period of its performance, unless affirmative proof establishing the contrary is presented. Similarly, in the absence of such records, an employee performing any work on or in connection with the contract in a workweek shall be presumed to have continued to perform such work throughout the workweek, unless affirmative proof establishing the contrary is presented. Even where a contractor can segregate Government from non-Government work, it is necessary that the contractor comply with the requirements of section 6(e) of the FLSA discussed in § 4.160 .
29 C.F.R. §4.179