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Definitions and conventions

ISO 8601

ISO 8601 is the international standard governing the numeric representation of dates and times. One of the formats allowed by the standard is "yyyy-mm-dd hh:mm". I use ISO 8601 notation because, like the metric system, it makes calculations and comparisons easier, once you're used to it.

UTC

Coordinated Universal Time is the technically correct phrase for what most people call GMT (Greenwich Mean Time). The abbreviation UTC was chosen to be language-neutral; the French phrase is Temps Universel Coordonné. UTC is the basis for legal time throughout the world. Astronomers use closely related time scales such as UT1 and UT2. When I write UTC+5:30, for example, I mean a legal time that is constantly five hours and thirty minutes later than UTC. If the minutes are :00, they will be omitted.

DST

Daylight Saving Time is also known as Summer Time in most of the world. Many jurisdictions adjust their legal time by moving clocks forward during the local summer. The usual adjustment is one hour, but there are exceptions. The purpose of DST is to save energy by encouraging people to get up earlier during a period when the sun rises earlier.

Standard Time

Standard Time is legal time in a specific area of the world. Generally, if a certain area observes DST every year, Standard Time refers only to the time when DST is not in effect.

Local Mean Time

Local Mean Time is solar time at a particular location on the earth, averaged out over seasonal variations in the sun's position. This was the legal time in most places before standard time was introduced. To compute the offset (in hours) from UTC to Local Mean Time, take the longitude of the location in degrees, using negative numbers in the western hemisphere, and divide by 15. For example, the longitude of Kokomo, Indiana is 86° 08' West, or -86 8/60. Dividing by 15, the Local Mean Time at Kokomo is UTC-5:44:32.

Transition times

The "Duration of DST", "Start Time", "Final Time", and "Description" columns contain times at which a transition took place from one UTC offset to another, or from one DST rule to another. Such times are expressed as local clock times. For example, if daylight saving time ends at 2001-10-28 02:00 in a certain location, the "02:00" is DST, not standard time, at that spot. Exception: in the beginning of standard time, a country will normally have many different local times before the transition (each city observes LMT). After the transition, they will all be synchronized (observing standard time). Normally, the transition occurs throughout the whole country at a single moment, such as 00:00 standard time. This standard time will be the time shown.

ab initio

Latin for "from the beginning". Local Mean Time was the time standard used, by default, long before people were aware of different time standards. I use the phrase "ab initio" because I can't give a specific date.

Present

From the last specific date on into the future, until the authorities declare otherwise.

tz database

Since 1986 (or earlier), Arthur David Olson has maintained a site for time zone downloads. The site contains source code for time zone calculations that are part of the POSIX standard, and a set of worldwide time zone data that support those calculations. Thanks to contributions from volunteers around the world, this database is almost certainly the most complete and up-to-date source of time zone information. It is sometimes called the Olson database.

Sources of additional information

A Summary of the International Standard Date and Time Notation  is a personal page with a thorough and reliable explanation of ISO 8601.
Sources for Time Zone and Daylight Saving Time Data  is a personal page with many useful links, closely related to the Olson tz database.
This  is the site for downloading the Olson tz database.
About Daylight Saving Time  is a short explanation and history of standard time and daylight saving time.
A Few Facts Concerning GMT, UT, and the RGO  explains the distinctions among GMT, UTC, UT1, UT2, and so on.
What is Universal Time?  is the U.S. Naval Observatory's explanation.
Time zones  is an astrology site with many world time zone names.

What is a time zone?

Some books say that the earth is divided into 24 time zones, each 15° wide, and centered on a series of meridians starting at the Greenwich Meridian (0°). This is, of course, an oversimplification. Actual time zones have been modified from this theoretical ideal in various ways. The boundaries of time zones deviate from the prescribed meridians almost everywhere. Several time zones differ from UTC by fractions of an hour. Currently, there are half- and quarter-hour offsets. Daylight saving time is another complicating factor. If two places both observe the same standard time, but one observes DST and the other one doesn't, are they in the same time zone or not?

In North America, according to popular usage, there are seven time zones: Newfoundland, Atlantic, Eastern, Central, Mountain, Pacific, and Alaska (the Hawaii-Aleutian zone is not part of North America). What defines each time zone is its standard time. Most of the time zones have some sections that do and some sections that don't observe DST. The parts that do observe DST may observe it on different schedules. For example, starting in 2001, the Central time zone includes places (most of Saskatchewan) that don't observe DST at all, places (Missouri, etc.) that observe it from April to October, and places (Mexico City) that observe it from May to September. Costa Rica has the same standard time as the Central time zone, but the name "Central Time" is not normally applied to places south of Mexico.

The North American usage, as described above, is echoed in Australia. The Australian zones are Eastern, Central, and Western. Members of the European Union are united in using the phrases "Central European Time" and "Eastern European Time" (or their equivalents in each language). Aside from those examples, standardized time zones with recognized names are very scarce. Most countries have only one time zone, and they usually refer to their legal time by the country's name, such as "Kenya Time".

To see what a time zone of this sort really is, consider what information is conveyed by an abbreviation like CST. In the North American system, it always represents an offset of UTC-6. (But there are times and places where the legal time is UTC-6, but the abbreviation CST would not be used. In fact, MDT - Mountain Daylight Time - also represents UTC-6.) It also implies that the time refers to a place in a particular region of North America, commonly marked on maps. It implies, too, that the date is between the last Sunday in October and the first Sunday in April - unless the place is in Saskatchewan where DST is not observed. In Saskatchewan, CST could be used any time of the year. In short, using the abbreviation "CST" in a North American context conveys the clear meaning "UTC-6", along with some hazy information that is unreliable for most purposes.

In the tz database, a time zone has a very precise meaning. It is an area, all or part of one country, in which the legal time has been uniform since 1970. Daylight saving time may be observed, but if so, every place in the time zone must advance and retard its clocks at the same time.

Time zone names in this database are binomial. The first part is the name of a continent or ocean. The second part, after a slash mark, is the name of a major city or other geographic feature in that time zone. The Central time zone in the North American system corresponds to four time zones in the tz database: America/Regina, America/Winnipeg, America/Chicago, and America/Mexico_City.

Are there standardized names and abbreviations for time zones?

There is the military standard , which uses 25 letters of the alphabet to represent time zones from UTC-12 to UTC+12. The various countries of the world currently observe offsets from UTC-11 to UTC+14, with some fractional hours among them, so the military standard won't meet all needs. (On the high seas, however, official time is zone time as determined by the longitude - see note below - and the military standard is perfectly suited.) The military standard also ignores DST. To use it, you have to make the mental adjustment for DST yourself.

Note: According to the authoritative book "Greenwich Time and the Longitude", by Derek Howse, a 1917 Anglo-French Conference on Timekeeping at Sea recommended that zone time be kept, according to the longitude of the ship's position. Most nations have adopted this rule.

The tz database contains time zone names and suggested abbreviations. The main drawback to this standard is that hardly anyone is aware that it exists.

Apart from the military standard and the tz database standard, there are only local or regional standards: North American, Australian, etc. These standards don't attempt to cover the whole world, and there may be conflicts between any two of them. For example, EST in North America means UTC-5. In Australia EST can mean UTC+10 or UTC+11, depending on the location and the date.