Western Hemisphere time zones. Universal Time (UT) is based on the time at the Royal Greenwich Observatory in England (upper right). The eastern part of the United States is offset by 5 hours from Universal Time. The central part of the USA and most of Mexico is 6 hours different from UT, while the West Coast of North America is 8 hours different. Time zones in South America are offset by 3 to 5 hours from UT.
Public domain image from Wikipedia.