Time zone

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A time zone is a region of the earth that has uniform standard time, usually referred to as the local time. By convention, time zones compute their local time as an offset from UTC (see also Greenwich Mean Time). Local time is UTC plus the current time zone offset for the considered location.

Introduction

Time zones are divided into standard and daylight saving (or summer). Daylight saving time zones (or summer time zones) include an offset (typically +1) for daylight saving time.

Standard time zones can be defined by geometrically subdividing the Earth's spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The local time in neighboring zones would differ by one hour. However, political boundaries, geographical practicalities, and convenience of inhabitants can result in irregularly-shaped zones. Moreover, in a few regions, half-hour or quarter-hour differences are in effect.

Before the adoption of time zones, people used local solar time. Originally this was apparent or true solar time, as shown by a sundial, and later it became mean solar time, as kept by most mechanical clocks. Mean solar time has days of equal length, but the difference between mean and apparent solar time, called the equation of time, averages to zero over a year.

The use of local solar time became increasingly awkward as railways and telecommunications improved, because clocks differed between places by an amount corresponding to the difference in their geographical longitude, which was usually not a convenient number. This problem could be solved by synchronizing the clocks in all localities, but in many places the local time would then differ markedly from the solar time to which people were accustomed. Time zones are a compromise, relaxing the complex geographic dependence while still allowing local time to approximate the mean solar time. There has been a general trend to set the boundaries of time zones west of their designated meridians in order to create a permanent daylight saving time effect. The increase in worldwide communication has further increased the need for interacting parties to communicate mutually comprehensible time references to one another.

Standard time zones

Until fairly recently, time zones were based on Greenwich Mean Time (GMT, also called UT1), the mean solar time at longitude 0° (the Prime Meridian). But as a mean solar time, GMT is defined by the rotation of the Earth, which is not constant in rate. So, the rate of atomic clocks was annually changed, or steered, to closely match GMT. In January 1972, however, atomic clock rates were fixed and predefined leap seconds replaced rate changes. This new time system is called Coordinated Universal Time (UTC). Leap seconds are inserted to keep UTC within 0.9 seconds of UT1. In this way, local times continue to correspond approximately to mean solar time, while the effects of variations in Earth's rotation rate are confined to simple step changes that can be more easily applied to obtain a uniform time scale (International Atomic Time or TAI). With the implementation of UTC, nations began to use it in the definition of their time zones instead of GMT. As of 2005, most but not all nations had altered the definition of local time in this way (though many media outlets fail to make a distinction between GMT and UTC). Further change to the basis of time zones may occur if proposals to abandon leap seconds succeed.

Due to daylight saving time, UTC is the local time at the Royal Observatory, Greenwich only between 01:00 UTC on the last Sunday in October and 01:00 UTC on the last Sunday in March. For the rest of the year, local time there is UTC+01, known in the United Kingdom as British Summer Time (BST). Similar circumstances apply in many other places.

Notation of time (ISO 8601)

UTC

If the time is in UTC, add a 'Z' directly after the time without a space. 'Z' is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".

UTC time is also known as 'Zulu' time, since 'Zulu' is the NATO phonetic alphabet word for 'Z'.

Time zone as offsets from UTC

Time zone are written as offset from UTC in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. So if the time being described is one hour ahead of UTC (such as the time in Berlin during the winter), the zone designator would be "+01:00", "+0100", or simply "+01". This is appended to the time in the same way that 'Z' was above. The offset from UTC changes with daylight saving time, e.g. a time offset in Chicago, would be "−06:00" for the winter (Central Standard Time) and "−05:00" for the summer (Central Daylight Time).

Abbreviations

Time zones are often represented by abbreviations such as "EST, WST, CST" but these are are not part of the international time and date standard ISO 8601 and their uses as sole designator for a time zone is not recommended.

Such designation can be ambiguous as some abbreviations may refer to different timezone in other continents for example EST WST CST can refer to Eastern, Western or Central, Summer Time or Standard Time in both North America and Australia.

Examples

These examples give the local time at various locations around the world at 12:00 UTC when daylight saving time (or summer time, etc.) is not in effect:

Location(s) Time zone Time
Baker Island, Howland Island (both uninhabited) UTC−12 00:00
Apia, Pago Pago UTC−11 01:00
Honolulu, Papeete UTC−10 02:00
Anchorage, Juneau UTC−09 03:00
Los Angeles, San Diego, San Francisco, Las Vegas, Vancouver, Tijuana, Seattle UTC−08 04:00
Boise, Calgary, Denver, El Paso, Hermosillo, Phoenix, Salt Lake City UTC−07 05:00
Chicago, Minneapolis, Mexico City, Houston, Nashville, Omaha, Winnipeg, San Salvador, San José, Guatemala City, Tegucigalpa, Panama City UTC−06 06:00
Toronto, Ottawa, Boston, New York City, Montreal, Washington D.C., Atlanta, Havana, Nassau, Kingston, Bogotá, Detroit, Lima, Quito UTC−05 07:00
Caracas UTC−04:30 07:30
Asunción, Bridgetown, Halifax, Roseau, Saint George, Santo Domingo, San Juan, Santiago, Port of Spain UTC−04 08:00
St. John's, Newfoundland and Labrador UTC−03:30 08:30
Buenos Aires, Montevideo, Rio de Janeiro, São Paulo UTC−03 09:00
Fernando de Noronha, South Georgia and the South Sandwich Islands UTC−02 10:00
Azores, Cape Verde UTC−01 11:00
Accra, Dakar, Monrovia, Dublin, Casablanca, London, Lisbon, Reykjavík, Canary Islands UTC 12:00
Berlin, Paris, Rome, Bern, San Marino, Praha, Brussels, Amsterdam, Wien, Belgrade, Stockholm, Oslo, Copenhagen, Luxembourg, Monaco, Madrid, Tunis UTC+01 13:00
Amman, Beirut, Istanbul, Athens, Cairo, Cape Town, Helsinki, Jerusalem, Bucharest, Sofia, Kiev, Tallinn, Riga, Vilnius UTC+02 14:00
Addis Ababa, Antananarivo, Baghdad, Dar es Salaam, Doha, Kampala, Kuwait City, Manama, Moscow, Mogadishu, Nairobi, Riyadh, Saint Petersburg, Sana'a UTC+03 15:00
Tehran UTC+03:30 15:30
Baku, Dubai, Muscat, Mauritius, Seychelles , Samara, Tbilisi UTC+04 16:00
Kabul UTC+04:30 16:30
Karachi, Maldives, Tashkent, Yekaterinburg UTC+05 17:00
Colombo, Chennai, New Delhi, Mumbai, Kolkata UTC+05:30 17:30
Kathmandu UTC+05:45 17:45
Almaty, Dhaka, Omsk UTC+06 18:00
Cocos Islands, Yangon UTC+06:30 18:30
Bangkok, Jakarta, Hanoi, Krasnoyarsk UTC+07 19:00
Beijing, Hong Kong, Irkutsk, Kuala Lumpur, Manila, Perth, Taipei, Singapore, Ulan Bator UTC+08 20:00
Pyongyang, Seoul, Tokyo, Osaka, Yakutsk UTC+09 21:00
Adelaide, Darwin UTC+09:30 21:30
Melbourne, Sydney, Vladivostok UTC+10 22:00
Magadan, Nouméa UTC+11 23:00
Auckland, Petropavlovsk-Kamchatsky, Suva UTC+12 00:00 (the following day)
Chatham Islands UTC+12:45 00:45 (the following day)
Nukuʻalofa UTC+13 01:00 (the following day)
Kiritimati UTC+14 02:00 (the following day)

Where the adjustment for time zones results in a time at the other side of midnight from UTC, then the date at the location is one day later or earlier.