- published: 10 May 2014
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The Pacific Time Zone observes standard time by subtracting eight hours from Coordinated Universal Time (UTC−8). The clock time in this zone is based on the mean solar time of the 120th meridian west of the Greenwich Observatory. During daylight saving time, its time offset is UTC−7.
In the United States and Canada, this time zone is generically called Pacific Time (PT). Specifically, it is Pacific Standard Time (PST) when observing standard time (Winter), and Pacific Daylight Time (PDT) when observing daylight saving time (Summer). Most of Canada uses daylight saving time. In Mexico the UTC−8 time zone is known as the Northwest Zone, which is synchronized with the U.S. PDT daylight saving schedule.
The largest city in the Pacific Time Zone is Los Angeles, California; the city's metropolitan area is the largest in the zone.
The zone is one hour ahead of the Alaska Time Zone, one hour behind the Mountain Time Zone and three hours behind the Eastern Time Zone.
The following states or areas are part of the Pacific Time Zone:
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. It is convenient for areas in close commercial or other communication to keep the same time, so time zones tend to follow the boundaries of countries and their subdivisions.
Most of the 40 time zones on land are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC−12 to UTC+14), but a few are offset by 30 or 45 minutes. Some higher latitude countries use daylight saving time for part of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west of the corresponding nautical time zones. This also creates a permanent daylight saving time effect.
Before the invention of clocks, people marked the time of day with apparent solar time (or "true" solar time) – for example, the time on a sundial – which was typically different for every settlement.
When well-regulated mechanical clocks became widespread in the early 19th century,[citation needed] each city began to use some local mean solar time. Apparent and mean solar time can differ by up to around 15 minutes (as described by the equation of time) due to the non-circular shape of the Earth's orbit around the sun. Mean solar time has days of equal length, and the difference between the two averages to zero after a year.