A time zone is a region of the Earth
that has adopted the same standard time, usually referred
to as the local time. Most 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 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 different 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.
Prior to 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. 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
Originally, 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 Greenwich, England 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 is UTC+1, known in the
UK 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. Here are some examples:
San Francisco, California, USA: UTC-8 (e.g.
if it is 12:00 UTC, then it is 04:00 in San Francisco)
Toronto, Ontario, Canada: UTC-5 (e.g. if it is 12:00 UTC,
then it is 07:00 in Toronto)
Stockholm, Sweden: UTC+1 (e.g. if it is 12:00 UTC, then it
is 13:00 in Stockholm)
Istanbul, Turkey: UTC+2 (e.g. if it is 12:00 UTC, then it
is 14:00 in Istanbul)
Mumbai, India: UTC+5:30 (e.g. if it is 12:00 UTC, then it
is 17:30 in Mumbai)
Tokyo, Japan: UTC+9 (e.g. if it is 12:00 UTC, then it is 21:00
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:
Cairo, Egypt: UTC+2 (e.g. if it is 23:00 UTC
on Monday, then the time in Cairo is 01:00, Tuesday)
Wellington, New Zealand: UTC+12 (e.g. if it is 21:00 UTC on
Monday, then the time in Wellington is 09:00, Tuesday)
New York City, USA: UTC-5 (e.g. if it is 02:00 UTC on Tuesday,
then the time in NY is 21:00, Monday)
Honolulu, Hawaii, USA: UTC-10 (e.g. if it is 06:00 UTC on
Tuesday, then the time in Honolulu is 20:00, Monday)
The time zone adjustment for a specific location may vary
due to the use 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.
Greenwich Mean Time (GMT) was established
in 1675 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 Greenwich Observatory.
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 . 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 Pittsburgh main station used six different times!
The confusion for travellers making a long journey involving
several changes of train can be imagined.
A system of one-hour standard time zones for
American railroads was first proposed by Charles F. Dowd about
1863. He proposed it while teaching teenage girls, but without
publishing anything. He did not even consult railroad officials
until 1869. In 1870, he proposed four ideal time zones (having
north-south borders), the first centered on Washington, DC,
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
put either forward or back as standard time noon was reached
within each time zone, east to west. The zones were named
Intercolonial, Eastern, Central, Mountain, and Pacific. Within
one year, 85% of all cities having populations over 10,000,
about 200 cities, were using standard time. A notable exception
was Detroit, Michigan, which kept local time until 1900, then
vacillated between Central Standard Time, local mean time,
and Eastern Standard Time until it settled on EST by ordinance
during May 1915, which was ratified by popular vote during
August 1916. This hodgepodge ended when Standard zone time
was formally adopted by the U.S. Congress in 1918.
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."
evertheless, 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 original conceived. The State of 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. The island of
Newfoundland, India, and parts of Australia use half-hour
deviations from standard time, and some nations use quarter-hour
Additionally, Australia does not apply Daylight
Saving Time uniformly across the country. Consequently, South
Australia, which, on standard time, observes the same time
as the Northern Territory but is 30 minutes behind Queensland,
New South Wales, Victoria, and Tasmania, and 90 minutes ahead
of Western Australia when Standard time; when DST is in effect,
is still 30 minutes behind NSW, Victoria and Tasmania, but
is 30 minutes AHEAD of Queensland (which is to the east),
one hour ahead of the Northern Territory (to the north), and
2.5 hours ahead of Western Australia, instead of 1.5 hours.
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 zonehe 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 AM
(except J) to the east and NY to the west (J may be
assigned to local time in non-nautical applications). These
were to be vocalized using a phonetic alphabet which included
Zulu for GMT, leading sometimes to the use of the term "Zulu
Time". "Zulu Time" should not be confused with
South Africa Standard Time (UTC+2), the time zone where Zulu
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 below.
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.
In terms of the largest number of time zones,
Russia is first, with eleven time zones, including Kaliningrad
on the Baltic Sea. The United States is second with nine time
zones, six for states and three more for possessions. Canada
is third with six time zones. Possessions of the United Kingdom
and France may increase their number of time zones.
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. The next largest
country with only one time zone is India (UTC+5:30). 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° 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
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).
Most modern computer operating systems include
information about time zones, including the capability to
automatically change the local time when daylight savings
starts and finishes (see the article on Daylight saving time
for more details on this aspect).
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.
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.
While most application software will use the
underlying operating system for timezone information, Java,
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.