The time and date recorded in the deck logs are local—as a ship moves east/west, the chronometers are advanced/retarded, and, at least in my time aboard ship, an announcement is made about the new time zone the ship is in.
As for the International Date Line, the saying “Sunday in San Francisco, Monday in Manila” still holds. It’s a personal choice if a sailor wants to keep a CONUS time—if only to remind them of the time if they call home from the next port. For operational reasons, if ships or commands are coordinating activities across multiple time zones, they will use Greenwich Mean Time (GMT, or Zulu Time) to ensure everyone is on the same page.
I hope you find this information useful.
Dear Mr. Ross,
Thank you for posting your request on History Hub!
US Navy ships use the local time and date for all deck log entries. If the ship crosses the international date line, the dates are adjusted forward or backward, depending on the direction of travel. This can result in days apparently being skipped or repeated in the sequence of deck log entries.
You may also be interested in the History Hub blogs Vessel & Station Log Books and Update on Availability of Vietnam Era (1956-1978) US Navy Deck Logs which provides information about deck logs available at the National Archives.
We hope this is helpful. Best of luck with your research!