A uniform time keeping system has been adopted for use at sea to ensure that all vessels within certain defined limits of longitude keep the same time. The world has been divided longitudinally into 24 zones of 15° each with the system centred on the meridian of Greenwich. The centre division lies between the meridians of 7½° east and 7½° west and is designated as zone 0. The zones lying to the eastward are numbered consecutively from 1 to 12 with a positive (+) prefix and those lying to westward also from 1 to 12 with a negative (-) prefix. The 12th zone is divided centrally by the 180° meridian and both prefixes (+) and (-) appear in this zone, their position depending on the “date line”. The zones are also given an alphabetical letter to identify them. The Greenwich zone is zone “Z”. The zones to the east are allocated the letters from “A” to “M” and omitting “J”. The zones to the west are allocated the letters “N” to “Y”.

Time zones.