Sundials preceded mechanical clocks. So before standard time zones were proclaimed, every locality determined its own time by daily resetting its clocks to 12 noon when dials showed the sun to be directly overhead. Each community, therefore, had clocks that differed from other localities east and west of it, but matched the communities north and south of it. Clearly, this wasn't a good solution.
Time zones were created by dividing the Earth's 360 degrees of longitude by 24 hours. That yielded 24 zones encircling Earth. Each zone is 15 degrees wide, stretches from pole to pole, and takes one hour to pass under the sun.
The reference point for this system is the prime meridian (0° longitude). Early English astronomers arbitrarily selected the meridian that runs from pole to pole through the original site of the Royal Observatory in Greenwich, England — and Greenwich Mean Time (GMT) was born. Gradually it was accepted by international convention, and it became the standard to which clocks everywhere would conform. Now all other longitudes are reckoned from this prime meridian. (Some nations, objecting to this England-centered system, established their own prime meridians.)
When railroads expanded across the various continents and required consistent timetables, governments created standardized time zones. In each 15-degree-wide time zone, the sun's zenith, or highest point, at mid-longitude in that zone became 12 noon for the entire zone.
The U.S. Congress established four time zones in 1883 and gave the Interstate Commerce Commission the task of governing them. These, and daylight saving time, are regulated today by the U.S. Department of Transportation. Five more zones were later added to U.S. time, so now there are nine. (Russia is in the most time zones, with 11.) The abbreviation of each is shown below (with its difference, or offset, from the prime meridian in England).
Four are within the contiguous United States:
| Name | Symbol |
From Greenwich |
| Eastern | EST |
(–5 hrs) |
| Central | CST |
(–6 hrs) |
| Mountain | MST |
(–7 hrs) |
| Pacific | PST |
(–8 hrs) |
In the Atlantic Ocean, there is one (for Puerto Rico & the U.S. Virgin Islands):
| Name | Symbol |
From Greenwich |
| Atlantic | AST |
(–4 hrs) |
In the Pacific Ocean, there are four:
| Name | Symbol |
From Greenwich |
| Alaska | AKST |
(–9 hrs) |
| Hawaii-Aleutian | HAST * |
(–10 hrs) |
| Samoa** | SST |
(–11 hrs) |
| Chamorro*** | ChST |
(+10 hrs) |
* Unofficially Hawaii standard time, HST
** For American Samoa
***
For Guam and the Northern Mariana Islands, west of the international date line
Because the Earth rotates from east to west, time zones east of Greenwich pass under the sun before Greenwich does. So their time zones are designated with plus signs to show that clocks there are ahead of (later than) Greenwich time. Zones to the west have negative signs because they are behind (earlier than) Greenwich time.
Why should you care what hour it is in England? Because that determines the time in other lands. If, for example, you're in St. Louis, Mo., (the Central or –6 zone) and decide at noon to call friends in Sydney, Australia (+9 zone), it would be thoughtful of you to calculate the time difference (6 + 9 = 15 hours) before you go waking them up at 3 a.m. their time.
For more information:
The official U.S. time, in any zone
Information about daylight saving time
The time-change controversy in Indiana, which is technically in two time zones
A brief history of international time zones
Information on Greenwich Mean Time