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Ais a region on Earth, more or less bounded by lines of longitude, that has a uniform, legally mandated standard time, usually referred to as the local time. By convention, the 24 main time zones on Earth compute their local time as an offset from UTC (see also Greenwich Mean Time), each time zone boundary being ostensibly 15 degrees east or west of the preceding one. The reference point for UTC is the Greenwich Meridian (the Prime Meridian), which has a longitude of 0°. Local time is UTC plus the current time zone offset for the location in question. In theory, the increase proceeds eastward from the eastern boundary of the UTC time zone centered on 0°, increasing by one hour for each 15°, up to the International Date Line (longitude 180°). A corresponding one hour decrease relative to UTC occurs every 15° heading westward from the western boundary of the UTC time zone, up to the International Date Line.
An effective way to tag timezone information is to add it to administration boundaries of countries or in the case of larger countries smaller administrative boundaries such as states and territories. (Do not create stand-alone boundaries for time zones in OSM.)