29 C.F.R. § 4.146
A contractor's obligation to observe the provisions of the Act arises on the date the contractor is informed that award of the contract has been made, and not necessarily on the date of formal execution. However, the contractor is required to comply with the provisions of the Act and regulations thereunder only while the employees are performing on the contract, provided the contractor's records make clear the period of such performance. (See also § 4.179 .) If employees of the contractor are required by the contract to complete certain preliminary training or testing prior to the commencement of the contract services, or if there is a phase-in period which allows the new contractor's employees to familiarize themselves with the contract work so as to provide a smooth transition between contractors, the time spent by employees undertaking such training or phase-in work is considered to be hours worked on the contract and must be compensated for even though the principal contract services may not commence until a later date.
29 C.F.R. §4.146