Time zones can be defined by geometrically subdividing the Earth into 24 sections separated by 15° of longitude. The local time in neighboring zones would differ by one hour. However, political boundaries, geographical practicalities, and convenience of inhabitants result in irregularly-shaped zones. Moreover, in a few regions, half-hour or quarter-hour differences are in effect.
By convention, time zones compute their local time as an offset from GMT (UTC). Local time is GMT plus the current time zone offset for the considered location. Click on each time zone to get its GMT offset.
note: This map does not adjust for Daylight Saving Time (DST). For time zones observing DST, add an hour to their GMT offset to compute local time.
source:
http://www.barnabu.co.uk/files/kmz/timezone_clock_nl.kml