About UTC Timezones

Coordinated Universal Time (UTC) is used as the official world reference for time. Coordinated Universal Time replaced the use of Greenwich Mean Time (GMT) in 1972. Sometimes time zones are represented similar to UTC - 5h or GMT - 5h. In this example, the (-5h) refers to that time zone being five hours behind UTC or GMT and so forth for the other time zones. UTC +5h or GMT +5h would refer to that time zone being five hours ahead of UTC of GMT and so forth for the other time zones.

The usage of UTC and GMT is based upon a twenty four hour clock, similar to military time, and is based upon the 0° longitude meridian, referred to as the Greenwich meridian in Greenwich, England.

UTC is based on cesium-beam atomic clocks, with leap seconds added to match earth-motion time, where as Greenwich Mean Time is based upon the Earth's rotation and celestial measurements. UTC is also known as Zulu Time or Z time.

In areas of the United States that observe Daylight Saving Time, local residents move their clocks ahead one hour when Daylight Saving Time begins. As a result, their UTC or GMT offset would change from UTC -5h or GMT - 5h to UTC -4h or GMT - 4h. In places not observing Daylight Saving Time the local UTC or GMT offset will remain the same year round. Arizona, Puerto Rico, Hawaii, U.S. Virgin Islands and American Samoa do not observe Daylight Saving Time.

In the United States Daylight Saving Time begins at 2:00 a.m. local time on the second Sunday in March. On the first Sunday in November areas on Daylight Saving Time return to Standard Time at 2:00 a.m. The names in each time zone change along with Daylight Saving Time. Eastern Standard Time (EST) becomes Eastern Daylight Time (EDT), and so forth. A new federal law took effect in March 2007 which extends Daylight Saving Time by four weeks.

The United States uses nine standard time zones. From east to west they are Atlantic Standard Time (AST), Eastern Standard Time (EST), Central Standard Time (CST), Mountain Standard Time (MST), Pacific Standard Time (PST), Alaskan Standard Time (AKST), Hawaii-Aleutian Standard Time (HST), Samoa standard time (UTC-11) and Chamorro Standard Time (UTC+10). The following tables identify the standard time zone boundaries and the offsets.

Standard Timezone Boundaries Table

Timezone Offsets Table