29 CFR § 785.48 - Use of time clocks.
(a) Differences between clock records and actual hours worked. Time clocks are not required. In those cases where time clocks are used, employees who voluntarily come in before their regular starting time or remain after their closing time, do not have to be paid for such periods provided, of course, that they do not engage in any work. Their early or late clock punching may be disregarded. Minor differences between the clock records and actual hours worked cannot ordinarily be avoided, but major discrepancies should be discouraged since they raise a doubt as to the accuracy of the records of the hours actually worked.
(b) “Rounding” practices. It has been found that in some industries, particularly where time clocks are used, there has been the practice for many years of recording the employees' starting time and stopping time to the nearest 5 minutes, or to the nearest one-tenth or quarter of an hour. Presumably, this arrangement averages out so that the employees are fully compensated for all the time they actually work. For enforcement purposes this practice of computing working time will be accepted, provided that it is used in such a manner that it will not result, over a period of time, in failure to compensate the employees properly for all the time they have actually worked.