World Time Zone
Earth is split up into a number of timezones. Most timezones are exactly one hour apart, and by convention compute their localtime as an offset from UTC or GMT. In many locations these offsets vary twice yearly due to DST transitions.
A timezone is 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 localtime. By convention, the 24 main timezones on Earth compute their localtime as an offset from UTC, each timezone 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°. Localtime is UTC plus the current timezone offset for the location in question. A corresponding one hour decrease relative to UTC occurs every 15° heading westward from the western boundary of the UTC timezone, up to the International Date Line.