I need an explanation with GMT, EST, UTC, timezone related?

Question

I need to understand the whole point, the big picture briefly. All about timezone

and the more specific:
It starts from where?
When I know the time of one country how to practically count/know the other country time?


10 pts. for good or detailed answer

Answer 1

Suppose you are standing on the North Pole, sometime in the summer so that the sun is above the horizon. You see the sun in front of you, and observe it moving to the right. All the meridians merge under your feet, and the sun will cross them in turn, moving fifteen degrees each hour. It will be local solar noon under the sun; as the sun moves, so will noon. It is clear that time, as defined by the sun, will vary from place to place, and that to make time more manageable, a zone system, in which all clocks in a zone would be similarly set, would be useful. The basics of the design are:
- There is a prime meridian, against which all other meridians are measured. By convention, it runs through the Naval Observatory in Greenwich, England.
- There must be a meridian at which the day begins. At any given time, there must be times at different places belonging to two different days. The boundary, which mostly follows the 180 meridian but has some wiggles for some island groups, is the International Date Line.
- The original universal time was Greenwich Mean Time. Because the earth's orbit is not quite circular, the duration of the time between succesive solar noons varies throughout the year. GMT is contrived to flatten the bumps, so that the sun is at the same place at the beginning of each year. It has been supplanted by Universal Coordinated Time, because there are irregularities in the earth's rotation and the desired accuracy can be had only with special clocks, which keep time precisely to a few parts in a trillion. Because the earth's rotation is slowing due to the tides, a leap second is added to the UTC about once a year.

The various time zones can be designated by the difference between them and GMT or UTC. Eastern standard time is five hours earlier than GMT, or GMT-5. Pacific Standard Time is GMT-8. Paris and Berlin are on GMT+1. By subraction, we can find that the difference between EST and Paris time is 6 hours.

Answer 2

I excerpted the part about Greenwich Mean Time, but you need to read the articles I've linked to for the full story:

Originally, the local time at the Royal Observatory, Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the widespread use of Greenwich Mean Time (GMT) in order to set local clocks. This location was chosen because by 1884 two-thirds of all charts and maps already used it as their Prime Meridian. In 1929, the term Universal Time (UT) was introduced to refer to GMT with the day starting at midnight. Until the 1950s, broadcast time signals were based on UT, and hence on the rotation of the Earth.

Answer 3

2000 Gmt To Est

Answer 4

A time zone is a region of the Earth that has adopted the same standard time, usually referred to as the local time. Most adjacent time zones are exactly one hour apart, and by convention compute their local time as an offset from Greenwich Mean Time .


Standard Time Zones of the World as of 2005. (Some time zones have changed since then)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 neighbouring zones is then exactly one hour different. However, political and geographical practicalities can result in irregularly-shaped zones that follow political boundaries or that change their time seasonally (as with daylight saving time), as well as being subject to occasional redefinition as political conditions change.

There are variations of the definitions of time zone which generally fall into two meanings: a time zone can represent a region where the local time is some fixed offset from a global reference (usually UTC), or a time zone can represent a region throughout which the local time is always consistent even though the offset may fluctuate seasonally.

Before the adoption of time zones, people used local solar time (originally apparent solar time, as with a sundial; and, later, mean solar time). Mean solar time is the average over a year of apparent solar time. Its difference from apparent solar time is the equation of time.

This 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 then in many places the local time would differ markedly from the solar time to which people are accustomed. Time zones are thus 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 push the boundaries of time zones further 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.

Earlier, time zones based their time 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. But on January 1, 1972 it became fixed, using predefined leap seconds instead of rate changes. This new time system is 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 easily subtracted if a uniform time scale (International Atomic Time or TAI) is desired. 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 have 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 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+1, known in the United Kingdom as British Summer Time (BST). Similar circumstances apply in many places.

The definition for time zones can be written in short form as UTC±n (or GMT±n), where n is the offset in hours.

Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich in England. It is now often used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, although strictly UTC is an atomic time scale which only approximates GMT in the old sense. It is also used to refer to Universal Time (UT), which is the astronomical concept that directly replaced the original GMT.

Noon Greenwich Mean Time is not necessarily the moment when the Sun crosses the Greenwich meridian (and reaches its highest point in the sky in Greenwich) because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (this discrepancy is known as the equation of time). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.

Historically the term GMT has been used with two different conventions for numbering hours. The old astronomical convention (before 1925) was to refer to noon as zero hours, whereas the civil and more modern convention is to refer to midnight as zero hours. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours

Coordinated Universal Time (UTC) is a high-precision atomic time standard. UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the earth's slowing rotation and other discrepancies. Leap seconds allow UTC to closely track Universal Time (UT), a time standard based not on the uniform passage of seconds, but on Earth's angular rotation.

Time zones around the world are expressed as positive or negative offsets from UTC. As the zero-point reference, UTC is also referred to as Zulu time (Z). UTC is often referred to as Greenwich Mean Time when describing time zones, although strictly speaking, it is only an approximation.

Answer 5

A time zone is a region of the Earth that has adopted the same standard time, usually referred to as the local time. Most adjacent time zones are exactly one hour apart, and by convention compute their local time as an offset from Greenwich Mean Time (see also UTC).


Standard Time Zones of the World as of 2005. (Some time zones have changed since then)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 neighbouring zones is then exactly one hour different. However, political and geographical practicalities can result in irregularly-shaped zones that follow political boundaries or that change their time seasonally (as with daylight saving time), as well as being subject to occasional redefinition as political conditions change.

There are variations of the definitions of time zone which generally fall into two meanings: a time zone can represent a region where the local time is some fixed offset from a global reference (usually UTC), or a time zone can represent a region throughout which the local time is always consistent even though the offset may fluctuate seasonally.

Before the adoption of time zones, people used local solar time (originally apparent solar time, as with a sundial; and, later, mean solar time). Mean solar time is the average over a year of apparent solar time. Its difference from apparent solar time is the equation of time.

This 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 then in many places the local time would differ markedly from the solar time to which people are accustomed. Time zones are thus 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 push the boundaries of time zones further 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
Earlier, time zones based their time 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. But on January 1, 1972 it became fixed, using predefined leap seconds instead of rate changes. This new time system is 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 easily subtracted if a uniform time scale (International Atomic Time or TAI) is desired. 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 have 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 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+1, known in the United Kingdom as British Summer Time (BST). Similar circumstances apply in many places.

The definition for time zones can be written in short form as UTC±n (or GMT±n), where n is the offset in hours. These examples give the local time at various locations at 12:00 UTC when daylight saving time (or summer time, etc.) is not in effect:

San Francisco, California, United States: UTC-8; 04:00
Toronto, Ontario, Canada: UTC-5; 07:00
Stockholm, Sweden: UTC+1; 13:00
Cape Town, South Africa: UTC+2; 14:00
Mumbai, India: UTC+5:30; 17:30
Tokyo, Japan: UTC+9; 21:00
Melbourne, Australia: UTC+10; 22:00
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. Some examples when UTC is 23:00 on Monday when daylight saving time is not in effect:

Cairo, Egypt: UTC+2; 01:00 on Tuesday
Wellington, New Zealand: UTC+12; 11:00 on Tuesday
Some examples when UTC is 02:00 on Tuesday when daylight saving time is not in effect:

New York City, New York, United States: UTC-5; 21:00 on Monday
Honolulu, Hawaii, United States: UTC-10; 16:00 on Monday
The time-zone adjustment for a specific location may vary because of Daylight Saving Time. For example New Zealand, which is usually UTC+12, observes a one-hour daylight saving time adjustment during the southern hemisphere summer, resulting in a local time of UTC+13.

See also Sidereal time.


[edit] History
Greenwich Mean Time (GMT) was established in 1675, when the Royal Observatory was built, as an aid to determine longitude at sea by mariners. The first time zone in the world was established by British railways on December 1, 1847 — with GMT hand-carried on chronometers. About August 23, 1852, time signals were first transmitted by telegraph from the Royal Observatory, Greenwich. Even though 98% of Great Britain's public clocks were using GMT by 1855, it was not made Britain's legal time until August 2, 1880. Some old clocks from this period have two minute hands — one for the local time, one for GMT [1]. This only applied to the island of Great Britain, and not to the island of Ireland.

On November 2, 1868, New Zealand (then a British colony) officially adopted a standard time to be observed throughout the colony, and was perhaps the first country to do so. It was based on the longitude 172° 30' East of Greenwich, that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time.

Timekeeping on the American railroads in the mid nineteenth century was somewhat confused. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad's train schedules were published using its own time. Some major railroad junctions served by several different railroads had a separate clock for each railroad, each showing a different time; the main station in Pittsburgh, Pennsylvania, for example, kept six different times. The confusion for travellers making a long journey involving several changes of train can be imagined.

Charles F. Dowd proposed a system of one-hour standard time zones for American railroads about 1863, although he published nothing on the matter at that time and did not consult railroad officials until 1869. In 1870, he proposed four ideal time zones (having north–south borders), the first centered on Washington, D.C., but by 1872 the first was centered 75°W of Greenwich, with geographic borders (for example, sections of the Appalachian Mountains). Dowd's system was never accepted by American railroads. Instead, U.S. and Canadian railroads implemented their own version on Sunday, November 18, 1883, also called "The Day of Two Noons", when each railroad station clock was reset as standard-time noon was reached within each time zone. The zones were named Intercolonial, Eastern, Central, Mountain, and Pacific. Within one year, 85% of all cities with populations over 10,000, about 200 cities, were using standard time. A notable exception was Detroit, Michigan (which is nearly half-way between the meridians of eastern time and central time, though actually a little closer to central), which kept local time until 1900, then tried Central Standard Time, local mean time, and Eastern Standard Time before a May 1915 ordinance settled on EST and was ratified by popular vote in August 1916. This hodgepodge ended when Standard zone time was formally adopted by the U.S. Congress on 19 March 1918 as the Standard Time Act.

Time zones were first proposed for the entire world by Canada's Sir Sandford Fleming in 1876 as an appendage to the single 24-hour clock he proposed for the entire world (located at the center of the Earth and not linked to any surface meridian). In 1879 he specified that his universal day would begin at the anti-meridian of Greenwich (now called 180°), while conceding that hourly time zones might have some limited local use. He continued to advocate his system at subsequent international conferences. In October 1884, the International Meridian Conference did not adopt his time zones because they were not within its purview. The conference did adopt a universal day of 24 hours beginning at Greenwich midnight, but specified that it "shall not interfere with the use of local or standard time where desirable".

Nevertheless, most major countries had adopted hourly time zones by 1929. Today, all nations use standard time zones for secular purposes, but they do not all apply the concept as originally conceived. Israel, for example, legally starts the day at 6:00 PM instead of midnight—so, the international date 1 January begins at what most other countries call 6:00 PM on 31 December. Newfoundland, India, Iran, Afghanistan, Burma, the Marquesas, as well as parts of Australia use half-hour deviations from standard time, and some nations, such as Nepal and the Chatham Islands use quarter-hour deviations.


[edit] Nautical time zones
Before 1920, all ships kept local apparent time on the high seas by setting their clocks at night or at the morning sight so that, given the ship's speed and direction, it would be 12 o'clock when the Sun crossed the ship's meridian (12 o'clock = local apparent noon). During 1917, at the Anglo-French Conference on Time-keeping at Sea, it was recommended that all ships, both military and civilian, should adopt hourly standard time zones on the high seas. A ship within the territorial waters of any nation would use that nation's standard time. The captain was permitted to change his ship's clocks at a time of his choice following his ship's entry into another time zone—he often chose midnight. These zones were adopted by all major fleets between 1920 and 1925 but not by many independent merchant ships until World War II.

Time on a ship's clocks and in a ship's log had to be stated along with a "zone description", which was the number of hours to be added to zone time to obtain GMT, hence zero in the Greenwich time zone, with negative numbers from −1 to −12 for time zones to the east and positive numbers from +1 to +12 to the west (hours, minutes, and seconds for nations without an hourly offset). These signs are opposite to those given below because ships must obtain GMT from zone time, not zone time from GMT. All zones were pole-to-pole staves 15° wide, except −12 and +12 which were each 7.5° wide, with the 180° meridian separating them. Unlike the zig-zagging land-based International Date Line, the nautical International Date Line follows 180° except where it is interrupted by territorial waters and the lands they border, including islands. About 1950, a letter suffix was added to the zone description, assigning Z to the Zero Zone, and A–M (except J) to the east and N–Y to the west (J may be assigned to local time in non-nautical applications; zones M and Y have the same clock time but differ by 24 hours: a full day). These were to be vocalized using a phonetic alphabet which included Zulu for GMT, leading sometimes to the use of the term "Zulu Time". The Greenwich time zone runs from 7.5°W to 7.5°E Longitude, while zone A runs from 7.5°E to 22.5°E Longitude, etc.

These nautical letters have been added to some time zone maps, like the map of Standard Time Zones by Her Majesty's Nautical Almanac Office (NAO), which extended the letters by adding an asterisk (*) or dagger (†) for areas that do not use a nautical time zone, and a double dagger (‡) for areas that do not have a legal standard time (Greenland's ice sheet and all of Antarctica. The United Kingdom specifies UTC-3 for the Antarctic Peninsula, but no other country recognizes that). They conveniently ignore any zone that does not have an hour or half-hour offset, so a double dagger (‡) has been co-opted for these zones in the list of time zones.

In maritime usage, GMT retains its historical meaning of UT1, the mean solar time at Greenwich. UTC, atomic time at Greenwich, is too inaccurate, differing by as much as 0.9 s from UT1, creating an error of 0.4 km in longitude at the equator. However, DUT can be added to UTC to correct it to within 50 ms of UT1, reducing the error to only 20 m.


[edit] Skewing of zones

Spain and France(in red) in relation to Greenwich Mean Time (in purple)Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian located in the middle of that zone with boundaries located 7+1⁄2 degrees east and west of the meridian. In practice, zone boundaries are often drawn much farther to the west with often irregular boundaries, and some locations base their time on meridians located far to the east.

For example, even though the Prime Meridian (0°) passes through Spain and France, they use the mean solar time of 15 degrees east (Central European Time) rather than 0 degrees (Greenwich Mean Time).

The tendency to draw time zone boundaries far to the west of their meridians allows greater utilization of more daylight in the afternoon hours. Many of these locations also use daylight saving time which means that solar noon could occur later than 2:00 P.M. by the clock.


[edit] Trivia
Russia has the largest number of time zones, eleven, including Kaliningrad on the Baltic Sea. In terms of distinct time zones, the United States is tied with Canada for second with six, while Australia has seven time zones that always overlap to some degree.
If the possessions of the United Kingdom, the United States and France are included it increases the number of time zones in each. Taking into account overseas territories and possessions, France has the most, with over twelve time zones, followed by Australia with ten, the United States with nine and the United Kingdom with over eight.
In terms of area, China is the largest country with only one time zone (UTC+8), although before the Chinese Civil War in 1949 China was separated into five time zones. China also has the widest spanning time zone.
Stations in Antarctica generally keep the time of their supply bases, thus both the Amundsen-Scott South Pole Station (U.S.) and McMurdo Station (U.S.) use New Zealand time (UTC+12 southern winter, UTC+13 southern summer).
The 27° N latitude passes back and forth across time zones in South Asia. Pakistan: +5, India +5:30, Nepal +5:45, India (Sikkim) +5:30, China +8:00, Bhutan +6:00, India (Arunachal Pradesh) +5:30, Myanmar +6:30. This switching was more odd in 2002, when Pakistan enabled Daylight Saving Time. Thus from west to east, time zones were: +6:00, +5:30, +5:45, +5:30, +8:00, +6:00, +5:30 and +6:30.
Because the earliest and latest time zones are 26 hours apart, any given calendar date exists at some point on the globe for 50 hours. For example, April 11 begins in time zone UTC+14 at 10:00 UTC April 10, and ends in time zone UTC-12 at 12:00 UTC April 12.
There are numerous places where several time zones meet, for instance at the tri-country border of Finland, Norway and Russia.
There are about 39 time zones instead of 24 (as popularly believed). This is due to fractional hour offsets and zones with offsets larger than 12 hours near the International Date Line. Some micronations may use offsets that are not recognized by all authorities.
The largest time gap along a political border is the 3.5 hour gap along the border of China (UTC +8) and Afghanistan (UTC+4:30).
One of the most unusual time zones is the Australian Central Western Time zone (CWST), which is in effect in a small strip of Western Australia from the border of South Australia west to E125.5°, just before Caiguna. It is 8¾ hours ahead of UTC (UTC+8:45) and covers an area of about 35,000 km², larger than Belgium, but has a population of about 200. Although unofficial, it is universally respected in the area, as, without it, the time gap, in standard time, at E129° (the WA/SA border) would be 2.5 hours. See Time in Australia.

[edit] Internet and computer systems
UTC is used on the Internet for meetings (i.e. IRC chats, news, shows and so on).

The format is based in the W3C Note "datetime".

On the other hand, most modern computer operating systems include information about time zones, including the capability to automatically change the local time when daylight saving starts and finishes (see the article on daylight saving time for more details on this aspect).


[edit] Unix
Main article: Unix time
Most Unix based systems, including Linux and Mac OS X, keep system time as UTC (Coordinated Universal Time). Rather than having a single timezone set for the whole computer, timezone offsets can vary for different processes. Standard library routines are used to calculate the local time based on the current timezone, normally supplied to processes through the TZ environment variable. This allows users in multiple timezones to use the same computer, with their respective local times displayed correctly to each user. Timezone information is most commonly stored in a timezone database known as zoneinfo (or sometimes tz or Olson format). In fact, many systems, including anything using the GNU C Library, can make use of this database.


[edit] Microsoft Windows
Windows based computer systems normally keep system time as local time in a particular time zone. A system database of timezone information includes the offset from UTC and rules that indicate the start and end dates for daylight savings in each zone. Application software is able to calculate the time in various zones, but there is no standard way for users from multiple zones to use a single server and have their own local time presented to them.


[edit] Smalltalk
Each Smalltalk dialect comes with its own built-in classes for dates, times and timestamps, only a few of which implement the DateAndTime and Duration classes as specified by the ANSI Smalltalk Standard. VisualWorks provides a TimeZone class that supports up to 2 annually-recurring offset transitions, which are assumed to apply to all years (same behavior as Windows time zones.) Squeak provides a Timezone class that does not support any offset transitions. Dolphin does not support time zones at all.

For full support of the Olson Time Zone Database (zoneinfo) in a Smalltalk application (including support for any number of annually-recurring offset transitions, and support for different intra-year offset transition rules in different years,) the third-party, open-source, ANSI-Smalltalk-compliant Chronos Date/Time Library is available for use with any of the following Smalltalk dialects: VisualWorks, Squeak or Dolphin.


[edit] Java
While most application software will use the underlying operating system for timezone information, Java language, from version 1.3.1, has maintained its own timezone database. This database (as well as the operating system database) will need to be updated whenever timezone rules change.[citation needed]

One unfortunate problem is that the date of the start of daylight saving time was altered by the New South Wales government for year 2000 to accommodate the 2000 Summer Olympics. This altered date was included with the Sun Java Virtual Machine.[citation needed]


[edit] Python
The standard module datetime stores and operates on the timezone information class tzinfo. The third party pytz module provides access to the full zoneinfo database.


[edit] Databases
Some databases allow storage of a datetime type having time zone information.


[edit] Oracle
Oracle Database is configured with a database time zone, and connecting clients are configured with session time zones. Oracle Database uses two data types to store time zone information:

TIMESTAMP WITH TIME ZONE
Stores date and time information with the offset from UTC
TIMESTAMP WITH LOCAL TIME ZONE
Stores date and time information with respect to the dbtimezone (which cannot be changed so long as there is a column in the db of this type), automatically adjusting the date and time from the stored time zone to the client's session time zone.

[edit] PostgreSQL
PostgreSQL uses the timestamptz datatype. It stores as UTC time, but operates on the current timezone, which could be different with every connection. Addition of the time interval includes daylight saving time.

Answer 6

Time zone

A time zone is a region of the Earth that has adopted the same standard time, usually referred to as the local time. Most adjacent time zones are exactly one hour apart, and by convention compute their local time as an offset from Greenwich Mean Time .

UTC

Coordinated Universal Time (UTC) is a high-precision atomic time standard. UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the earth's slowing rotation and other discrepancies. Leap seconds allow UTC to closely track Universal Time (UT), a time standard based not on the uniform passage of seconds, but on Earth's angular rotation.

Time zones around the world are expressed as positive or negative offsets from UTC. As the zero-point reference, UTC is also referred to as Zulu time (Z). UTC is often referred to as Greenwich Mean Time when describing time zones, although strictly speaking, it is only an approximation.





Standard Time Zones of the World as of 2005. (Some time zones have changed since then)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 neighbouring zones is then exactly one hour different. However, political and geographical practicalities can result in irregularly-shaped zones that follow political boundaries or that change their time seasonally (as with daylight saving time), as well as being subject to occasional redefinition as political conditions change.

There are variations of the definitions of time zone which generally fall into two meanings: a time zone can represent a region where the local time is some fixed offset from a global reference (usually UTC), or a time zone can represent a region throughout which the local time is always consistent even though the offset may fluctuate seasonally.

Before the adoption of time zones, people used local solar time (originally apparent solar time, as with a sundial; and, later, mean solar time). Mean solar time is the average over a year of apparent solar time. Its difference from apparent solar time is the equation of time.

This 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 then in many places the local time would differ markedly from the solar time to which people are accustomed. Time zones are thus 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 push the boundaries of time zones further 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

Earlier, time zones based their time 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. But on January 1, 1972 it became fixed, using predefined leap seconds instead of rate changes. This new time system is 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 easily subtracted if a uniform time scale (International Atomic Time or TAI) is desired. 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 have 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 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+1, known in the United Kingdom as British Summer Time (BST). Similar circumstances apply in many places.

The definition for time zones can be written in short form as UTC±n (or GMT±n), where n is the offset in hours. These examples give the local time at various locations at 12:00 UTC when daylight saving time (or summer time, etc.) is not in effect:

San Francisco, California, United States: UTC-8; 04:00
Toronto, Ontario, Canada: UTC-5; 07:00
Stockholm, Sweden: UTC+1; 13:00
Cape Town, South Africa: UTC+2; 14:00
Mumbai, India: UTC+5:30; 17:30
Tokyo, Japan: UTC+9; 21:00
Melbourne, Australia: UTC+10; 22:00
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. Some examples when UTC is 23:00 on Monday when daylight saving time is not in effect:

Cairo, Egypt: UTC+2; 01:00 on Tuesday
Wellington, New Zealand: UTC+12; 11:00 on Tuesday
Some examples when UTC is 02:00 on Tuesday when daylight saving time is not in effect:

New York City, New York, United States: UTC-5; 21:00 on Monday
Honolulu, Hawaii, United States: UTC-10; 16:00 on Monday
The time-zone adjustment for a specific location may vary because of Daylight Saving Time. For example New Zealand, which is usually UTC+12, observes a one-hour daylight saving time adjustment during the southern hemisphere summer, resulting in a local time of UTC+13.




History

Greenwich Mean Time (GMT) was established in 1675, when the Royal Observatory was built, as an aid to determine longitude at sea by mariners. The first time zone in the world was established by British railways on December 1, 1847 — with GMT hand-carried on chronometers. About August 23, 1852, time signals were first transmitted by telegraph from the Royal Observatory, Greenwich. Even though 98% of Great Britain's public clocks were using GMT by 1855, it was not made Britain's legal time until August 2, 1880. Some old clocks from this period have two minute hands — one for the local time, one for GMT [1]. This only applied to the island of Great Britain, and not to the island of Ireland.

On November 2, 1868, New Zealand (then a British colony) officially adopted a standard time to be observed throughout the colony, and was perhaps the first country to do so. It was based on the longitude 172° 30' East of Greenwich, that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time.

Timekeeping on the American railroads in the mid nineteenth century was somewhat confused. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad's train schedules were published using its own time. Some major railroad junctions served by several different railroads had a separate clock for each railroad, each showing a different time; the main station in Pittsburgh, Pennsylvania, for example, kept six different times. The confusion for travellers making a long journey involving several changes of train can be imagined.

Charles F. Dowd proposed a system of one-hour standard time zones for American railroads about 1863, although he published nothing on the matter at that time and did not consult railroad officials until 1869. In 1870, he proposed four ideal time zones (having north–south borders), the first centered on Washington, D.C., but by 1872 the first was centered 75°W of Greenwich, with geographic borders (for example, sections of the Appalachian Mountains). Dowd's system was never accepted by American railroads. Instead, U.S. and Canadian railroads implemented their own version on Sunday, November 18, 1883, also called "The Day of Two Noons", when each railroad station clock was reset as standard-time noon was reached within each time zone. The zones were named Intercolonial, Eastern, Central, Mountain, and Pacific. Within one year, 85% of all cities with populations over 10,000, about 200 cities, were using standard time. A notable exception was Detroit, Michigan (which is nearly half-way between the meridians of eastern time and central time, though actually a little closer to central), which kept local time until 1900, then tried Central Standard Time, local mean time, and Eastern Standard Time before a May 1915 ordinance settled on EST and was ratified by popular vote in August 1916. This hodgepodge ended when Standard zone time was formally adopted by the U.S. Congress on 19 March 1918 as the Standard Time Act.

Time zones were first proposed for the entire world by Canada's Sir Sandford Fleming in 1876 as an appendage to the single 24-hour clock he proposed for the entire world (located at the center of the Earth and not linked to any surface meridian). In 1879 he specified that his universal day would begin at the anti-meridian of Greenwich (now called 180°), while conceding that hourly time zones might have some limited local use. He continued to advocate his system at subsequent international conferences. In October 1884, the International Meridian Conference did not adopt his time zones because they were not within its purview. The conference did adopt a universal day of 24 hours beginning at Greenwich midnight, but specified that it "shall not interfere with the use of local or standard time where desirable".

Nevertheless, most major countries had adopted hourly time zones by 1929. Today, all nations use standard time zones for secular purposes, but they do not all apply the concept as originally conceived. Israel, for example, legally starts the day at 6:00 PM instead of midnight—so, the international date 1 January begins at what most other countries call 6:00 PM on 31 December. Newfoundland, India, Iran, Afghanistan, Burma, the Marquesas, as well as parts of Australia use half-hour deviations from standard time, and some nations, such as Nepal and the Chatham Islands use quarter-hour deviations.


Nautical time zones

Before 1920, all ships kept local apparent time on the high seas by setting their clocks at night or at the morning sight so that, given the ship's speed and direction, it would be 12 o'clock when the Sun crossed the ship's meridian (12 o'clock = local apparent noon). During 1917, at the Anglo-French Conference on Time-keeping at Sea, it was recommended that all ships, both military and civilian, should adopt hourly standard time zones on the high seas. A ship within the territorial waters of any nation would use that nation's standard time. The captain was permitted to change his ship's clocks at a time of his choice following his ship's entry into another time zone—he often chose midnight. These zones were adopted by all major fleets between 1920 and 1925 but not by many independent merchant ships until World War II.

Time on a ship's clocks and in a ship's log had to be stated along with a "zone description", which was the number of hours to be added to zone time to obtain GMT, hence zero in the Greenwich time zone, with negative numbers from −1 to −12 for time zones to the east and positive numbers from +1 to +12 to the west (hours, minutes, and seconds for nations without an hourly offset). These signs are opposite to those given below because ships must obtain GMT from zone time, not zone time from GMT. All zones were pole-to-pole staves 15° wide, except −12 and +12 which were each 7.5° wide, with the 180° meridian separating them. Unlike the zig-zagging land-based International Date Line, the nautical International Date Line follows 180° except where it is interrupted by territorial waters and the lands they border, including islands. About 1950, a letter suffix was added to the zone description, assigning Z to the Zero Zone, and A–M (except J) to the east and N–Y to the west (J may be assigned to local time in non-nautical applications; zones M and Y have the same clock time but differ by 24 hours: a full day). These were to be vocalized using a phonetic alphabet which included Zulu for GMT, leading sometimes to the use of the term "Zulu Time". The Greenwich time zone runs from 7.5°W to 7.5°E Longitude, while zone A runs from 7.5°E to 22.5°E Longitude, etc.

These nautical letters have been added to some time zone maps, like the map of Standard Time Zones by Her Majesty's Nautical Almanac Office (NAO), which extended the letters by adding an asterisk (*) or dagger (†) for areas that do not use a nautical time zone, and a double dagger (‡) for areas that do not have a legal standard time (Greenland's ice sheet and all of Antarctica. The United Kingdom specifies UTC-3 for the Antarctic Peninsula, but no other country recognizes that). They conveniently ignore any zone that does not have an hour or half-hour offset, so a double dagger (‡) has been co-opted for these zones in the list of time zones.

In maritime usage, GMT retains its historical meaning of UT1, the mean solar time at Greenwich. UTC, atomic time at Greenwich, is too inaccurate, differing by as much as 0.9 s from UT1, creating an error of 0.4 km in longitude at the equator. However, DUT can be added to UTC to correct it to within 50 ms of UT1, reducing the error to only 20 m.


Skewing of zones

Spain and France(in red) in relation to Greenwich Mean Time (in purple)Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian located in the middle of that zone with boundaries located 7+1⁄2 degrees east and west of the meridian. In practice, zone boundaries are often drawn much farther to the west with often irregular boundaries, and some locations base their time on meridians located far to the east.

For example, even though the Prime Meridian (0°) passes through Spain and France, they use the mean solar time of 15 degrees east (Central European Time) rather than 0 degrees (Greenwich Mean Time).

The tendency to draw time zone boundaries far to the west of their meridians allows greater utilization of more daylight in the afternoon hours. Many of these locations also use daylight saving time which means that solar noon could occur later than 2:00 P.M. by the clock.


Trivia
Russia has the largest number of time zones, eleven, including Kaliningrad on the Baltic Sea. In terms of distinct time zones, the United States is tied with Canada for second with six, while Australia has seven time zones that always overlap to some degree.
If the possessions of the United Kingdom, the United States and France are included it increases the number of time zones in each. Taking into account overseas territories and possessions, France has the most, with over twelve time zones, followed by Australia with ten, the United States with nine and the United Kingdom with over eight.
In terms of area, China is the largest country with only one time zone (UTC+8), although before the Chinese Civil War in 1949 China was separated into five time zones. China also has the widest spanning time zone.
Stations in Antarctica generally keep the time of their supply bases, thus both the Amundsen-Scott South Pole Station (U.S.) and McMurdo Station (U.S.) use New Zealand time (UTC+12 southern winter, UTC+13 southern summer).
The 27° N latitude passes back and forth across time zones in South Asia. Pakistan: +5, India +5:30, Nepal +5:45, India (Sikkim) +5:30, China +8:00, Bhutan +6:00, India (Arunachal Pradesh) +5:30, Myanmar +6:30. This switching was more odd in 2002, when Pakistan enabled Daylight Saving Time. Thus from west to east, time zones were: +6:00, +5:30, +5:45, +5:30, +8:00, +6:00, +5:30 and +6:30.
Because the earliest and latest time zones are 26 hours apart, any given calendar date exists at some point on the globe for 50 hours. For example, April 11 begins in time zone UTC+14 at 10:00 UTC April 10, and ends in time zone UTC-12 at 12:00 UTC April 12.
There are numerous places where several time zones meet, for instance at the tri-country border of Finland, Norway and

Russia.

There are about 39 time zones instead of 24 (as popularly believed). This is due to fractional hour offsets and zones with offsets larger than 12 hours near the International Date Line. Some micronations may use offsets that are not recognized by all authorities.
The largest time gap along a political border is the 3.5 hour gap along the border of China (UTC +8) and Afghanistan (UTC+4:30).
One of the most unusual time zones is the Australian Central Western Time zone (CWST), which is in effect in a small strip of Western Australia from the border of South Australia west to E125.5°, just before Caiguna. It is 8¾ hours ahead of UTC (UTC+8:45) and covers an area of about 35,000 km², larger than Belgium, but has a population of about 200. Although unofficial, it is universally respected in the area, as, without it, the time gap, in standard time, at E129° (the WA/SA border) would be 2.5 hours.