Day means, for the purpose of complying with the service requirements of this subchapter, 8 hours of watchstanding or day-working not to include overtime. On vessels authorized by 46 U.S.C. 8104 and 46 CFR 15.705, to operate a two-watch system, a 12-hour working day may be creditable as 1 1/2 days of service. On vessels of less than 100 GRT, a day is considered as 8 hours unless the Coast Guard determines that the vessel's operating schedule makes this criteria inappropriate; in no case will this period be less than 4 hours. When computing service on MODUs for any endorsement, a day of MODU service must be a minimum of 4 hours, and no additional credit is received for periods served over 8 hours. For cadet service on a training ship furnished by the Maritime Administration under 46 CFR 310.4, a day may be creditable as 1 1/2 days of service.
46 CFR § 10.107
With respect to part 16 of this subchapter only, if the definitions in paragraph (b) of this section differ from those set forth in 16.105, the definition set forth in 16.105 applies.