29 CFR 778.303: Retroactive pay increases.

Where a retroactive pay increase is awarded to employees as a result of collective bargaining or otherwise, it operates to increase the regular rate of pay of the employees for the period of its retroactivity. Thus, if an employee is awarded a retroactive increase of 10 cents per hour, he is owed, under the Act, a retroactive increase of 15 cents for each overtime hour he has worked during the period, no matter what the agreement of the parties may be. A retroactive pay increase in the form of a lump sum for a particular period must be prorated back over the hours of the period to which it is allocable to determine the resultant increases in the regular rate, in precisely the same manner as a lump sum bonus. For a discussion of the method of allocating bonuses based on employment in a prior period to the workweeks covered by the bonus payment, see § 778.209.