Zone Time
F5. The Reference Layer: Glossary, Units, Signals and Information SourcesDefinition
Standard time of a 15-degree time zone, used at sea.
Zone time is the standard time kept aboard, set to the central meridian of a 15-degree-wide time zone, so each zone differs from UTC by a whole number of hours. The zone is numbered by the hours added to local time to get UTC: zone +5 is 5 hours behind UTC (75 degrees W), zone -5 ahead. A ship crossing zones advances or retards clocks by an hour, usually at night, and the navigator logs the zone in use. Zone time keeps shipboard routine roughly aligned with the sun while UTC governs astronomical and radio work.