Caption: Due to the Earth's spherical shape and its rotation around its own axis, local noon – the moment in time when, for an observer at a specific geographic location, the Sun reaches its highest position in the sky varies with longitude. That is why a time coordinate, such as Universal Time (UT or UTC), might be of advantage for specifying moments in time in a unified way. But a global time coordinate will be out of synch with the local day-night rhythm in most locations on Earth. A compromise is to divide Earth into zones each comprising 15 degrees of longitude, and in each zone define time as UTC plus or minus an integer number of hours, with the offset chosen so as to make time match as closely with local time at the middle longitude of the time zone. In practice, political considerations have altered the time zone boundaries somewhat. When a country straddles several time zones, it is not uncommon for the country's government to choose one of those time zones to define the country's official time. This map, originally created by the CIA and updated by several Wikimedia Commons users, shows the current definitions of the world's time zones. The IAU OAE is not the original author of this map. The designations employed and the presentation of the material on this maps do not imply the expression of any opinion whatsoever on the part of the IAU or the IAU OAE concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.
Credit: Wikimedia Users UnaitxuGV, Heitordp and others based on a map created by the U.S. Central Intelligence Agency (CIA) credit link