Before the invention of clocks, the time of day was determined by apparent solar time. For example, a sundial was used to estimate the time based on the Sun's apparent position in the sky. When mechanical clocks became common in the 19th century, mean solar time was used. Apparent and mean solar time may differ by up to 15 minutes. Further advances led to the concept of standard time zones. Time zones are regions on the globe that observe the same standard time. The local time in a given time zone is defined by its offset from Coordinated Universal Time (UTC), which is the primary time standard used throughout the world. UTC time either changes backward or forward by one hour for every 15 degrees west or east of the prime meridian.
United States Time Zones
By law, time in the United States (US) and its possessions is divided into nine standard time zones. However, the 50 states are spread across six standard time zones: Pacific, Central, Mountain, Eastern, Alaska, and Hawaii. The other three time zones are used in US territories: the Atlantic Time Zone is followed in Puerto Rico and the US Virgin Islands; the Chamorro Time Zone is followed in Guam; and the Samoa Time Zone is followed in American Samoa. Most states observe Daylight Saving Time (DST), which begins at 0200 hours on the second Sunday of March and runs until 0159 hours on the first Sunday of November. DST is not observed in the states of Hawaii and Arizona, and some US territories including Puerto Rico, Guam, American Samoa, and the Virgin Islands. The time zones are defined by their hourly offset from UTC.
Time Zones Within the 50 US States
Hawaii-Aleutian Time Zone
The Hawaii-Aleutian Time Zone derives its name from two areas: Hawaii and the Aleutian Islands in Alaska. These two areas do not use daylight saving time. The time zone observes Hawaii-Aleutian Standard Time (HAST), which is obtained by subtracting 10 hours from UTC (UTC-10:00). Some of the major metropolitan areas under this time zone include Honolulu, Hilo, and Kapaa.
Alaska Time Zone
The Alaska Time Zone encompasses almost the entire state of Alaska. Standard time is observed in this time zone by subtracting 9 hours from UTC (UTC-09:00) and 8 hours during DST (UTC-08:00). The clock time in the Alaska Time Zone is based on the mean solar time at 135 degrees west of Greenwich Observatory.
Pacific Time Zone
The Pacific Time Zone (PT) encompasses the western states of the US, western Mexico, and western Canada. The standard time within this zone is observed by subtracting 8 hours from UTC (UTC-8). During DST, an offsite time of UTC-7 is used. PST is one hour ahead of Hawaii and Alaska Time Zones, and at least one hour behind the other time zones.
Mountain Time Zone
The Mountain Time Zone (MT) observes standard time by subtracting 7 hours from UTC (UTC-07:00) and 6 hours during DST (UTC-06:00). In the US, this time zone is generally known as Mountain Time (MT). Five states, namely Colorado, New Mexico, Wyoming, Utah, and Montana, are included within the Mountain Time Zone.
Central Time Zone
The Central Time Zone (CT) encompasses nine states in the US, the Canadian province of Manitoba, and most of Mexico. It is observed by subtracting six hours from UTC (UTC-06:00). Most of the zone uses DST during summer and changes to Central Daylight Time (CDT), which is 5 hours behind UTC.
Eastern Time Zone
The Eastern Time Zone (ET) encompasses most of the 22 states in the eastern United States. Places within this time zone are 5 hours behind UTC. The northern part of ET is advanced by one hour to 0300 hours EDT at 0200 hours EST on the first Sunday of March. On the first Sunday of November, the time is moved back by one hour at 0200 hours EDT to 0100 hours EST.