There are 9 standard time zones in the USA officially defined by federal law. The entire 50 states and the District of Columbia are situated in 6 main time zones.
- UTC-05:00 Eastern Time Zone, the most populous in the USA, includes the District of Columbia with the capital city of Washington and 23 states wholly or partially located within this zone. De facto the Eastern Time often is used as the USA official time.
- UTC-06:00 Central Time Zone, the biggest one by area, encompasses 20 states, but only 10 of them are entirely located within zone's boundaries, all remaining states have some exceptions.
- UTC-07:00 Mountain Time Zone includes 14 states, only 5 of them are wholly located in this zone, all others use the Mountain Time only in some parts of their territories.
- UTC-08:00 Pacific Time Zone covers 5 states - the entire states of California and Washington, most of Nevada and Oregon with some exceptions, and some northern parts of Idaho.
- UTC-09:00 Alaska Time Zone, as clearly observed in its name, includes the most of the territory of Alaska.
- UTC-10:00 Hawaii-Aleutian Time Zone covers Hawaii and western part of the Aleutian Islands belonging to Alaska.
Other 3 time zones are used by insular areas of the USA outside of the states.
- UTC-04:00 Atlantic Time Zone comprises the Puerto Rico and the United States Virgin Islands in the Caribbean.
- UTC-11:00 Samoa Time Zone is used in the American Samoa islands in the South Pacific.
- UTC+10:00 Chamorro Time Zone comprises the Guam and the Northern Mariana Islands, located in the northwestern part of the Pacific Ocean.
Nowadays, 48 of the US states use Daylight Saving Time (DST) in the summer period, the exceptions are Arizona, where most of territories do not change to DST (since 1967), and the entire Hawaii, which has never observed the summer time. All insular territories like Puerto Rico, US Virgin Islands, American Samoa, Guam and the Northern Mariana Islands have no DST.
Daylight Saving Time (DST) in 2015 - starts on Sunday 08 March 2015, 02:00 local time (clock by 1 hour ahead) - ends on Sunday 01 November 2015, 02:00 local time (clock by 1 hour back)