Time zone (ship)

A ship’s time zone is the whole-hour band of longitude a vessel keeps its clocks to at sea, so ship’s time stays roughly in step with the sun. The world is divided into 24 zones, each 15 degrees of longitude wide and centered on a standard meridian a multiple of 15 degrees, running 7 degrees 30 minutes either side of it. The master advances or retards the clocks by one hour when the ship crosses into the next zone, usually over the night watches. Each zone has a zone description, the whole number of hours that converts zone time to UTC. The International Date Line near 180 degrees longitude is where the date changes on an east-west crossing.