Clarifying some definitions:
• A day of sea service is any day that a mariner served upon a vessel in an assigned position in either the deck or engineering department of a vessel (not a passenger). The position may include duties such as: handling lines, being a lookout, steering the boat, and other navigational or propulsion functions.
Then here is a link to counting military time, which includes submarines.
https://www.dco.uscg.mil/Portals/9/NMC/pdfs/professional_qualifications/crediting_military_ss.pdf
Service is now counted at 60% of the total time aboard a ship. However, documenting and calculating gets far more complicated, in some ways ridiculously difficult to get the records and documentation.
And definition of a day:
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 11⁄2 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 11⁄2 days of service.
MODU is Mobile Offshore Drilling Unit