One of the 24 longitudinal bands into which the Earth is divided; or, any region of the Earth that keeps the same civil time. The 24 standard time zones are each 15° wide in longitude, and each is assigned a zone time that differs by 1 hour from the adjacent time zone, being 1 hour later to the east and 1 hour earlier to the west, until the International Date Line is reached when a whole day changes. The Earth’s prime time zone is centred on the meridian of Greenwich and its standard time is Universal Time. Large countries such as the USA, Russia, and Australia are divided into several time zones according to local geographical or political considerations rather than strict parallels of longitude.

http://astro.ukho.gov.uk/nao/miscellanea/WMTZ/ Map of current time zones at HM Nautical Almanac Office.