The world is divided into 24 time zones, each covering 15° of longitude. Zone Zero is 7.5° either side of the Greenwich meridian. The zones then move round the world east and west of Greenwich in 15° units, although some countries tweak them to fit local geography. They meet at the International Date Line. This more or less follows the 180° meridian and the eastern hemisphere is always one day ahead of the date in the western hemisphere.
Local time in a ship's log has its 'zone description', which is the number of hours that must be added or subtracted to find GMT. Time zones to the east of Greenwich cover the hours —1 to — 12 and those to the west +1 to +12. In the 1950s zone Zero was given the letter Z, zones to the east were labelled A through M (missing out J), and zones to the west labelled M through Y. However, the usual convention is to describe zone time as Z (or Zulu) plus or minus the number of hours that must be added or subtracted to find GMT.
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