29 CFR 785.16 Off duty.

(a) General. Periods during which an employee is completely relieved from duty and which are long enough to enable him to use the time effectively for his own purposes are not hours worked. He is not completely relieved from duty and cannot use the time effectively for his own purposes unless he is definitely told in advance that he may leave the job and that he will not have to commence work until a definitely specified hour has arrived. Whether the time is long enough to enable him to use the time effectively for his own purposes depends upon all of the facts and circumstances of the case.

(b) Truck drivers; specific examples. A truck driver who has to wait at or near the job site for goods to be loaded is working during the loading period. If the driver reaches his destination and while awaiting the return trip is required to take care of his employer's property, he is also working while waiting. In both cases the employee is engaged to wait. Waiting is an integral part of the job. On the other hand, for example, if the truck driver is sent from Washingtion, DC to New York City, leaving at 6 a.m. and arriving at 12 noon, and is completely and specifically relieved from all duty until 6 p.m. when he again goes on duty for the return trip the idle time is not working time. He is waiting to be engaged. (Skidmore v. Swift, 323 U.S. 134, 137 (1944); Walling v. Dunbar Transfer & Storage, 3 W.H. Cases 284; 7 Labor Cases para. 61,565 (W.D. Tenn. 1943); Gifford v. Chapman, 6 W.H. Cases 806; 12 Labor Cases para. 63,661 (W.D. Okla., 1947); Thompson v. Daugherty, 40 Supp. 279 (D. Md. 1941))