United States time zones are nine standard time zones covering the states and possessions of the United States, used in conjunction with state and national legislation regarding the use of Daylight Saving Time.
Daylight Saving Time (also called DST, or summer time) is the portion of the year in which a region's local time is advanced by (usually) one hour from its official standard time. It is most common in temperate regions.
"Daylight saving time" is a system intended to "save" daylight, as opposed to "wasting" time. The official time is adjusted forward during the spring and summer months, so that the active hours of work and school will better match the hours of daylight.
From east to west, these zones are:
Mountain Standard Time (MST; GMT−7; Zone T), which comprises roughly the states that include the Continental Divide and Rocky Mountains plus much of the Great Basin and the watershed of the Colorado River.
Atlantic Standard Time (AST, GMT−4, Zone Q), which comprises Puerto Rico and the U.S. Virgin Islands.
Samoa Standard Time (GMT−11, Zone X), which comprises American Samoa.
Chamorro Standard Time (GMT+10, Zone K), which comprises Guam and the Northern Mariana Islands.