Zone description

The zone description, ZD, is the whole number of hours that converts a ship’s zone time to UTC, the offset that labels each of the 24 time zones at sea. The navigation sign convention is the reverse of the everyday one: the ZD is positive in west longitude, where local time is behind UTC, and negative in east longitude, so that UTC equals zone time plus the zone description. Each zone is 15 degrees wide, centered on a standard meridian a multiple of 15 degrees, so the ZD changes by one each zone. Zone zero straddles Greenwich and keeps UTC. The navigator applies the ZD to keep ship’s clocks, time sights, and state an ETA in UTC.