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Standard Time Zones and Greenwich Mean Time


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                                 "Time is defined as what our clocks measure."[1]

The measurement of time has occupied scientists and technologists for centuries. There are many usages and definitions of "Time". The National Institute of Standards and Technology (NIST) define time as:

The designation of an instant on a selected time scale, used in the sense of time of day; or the interval between two events or the duration of an event, used in the sense of time interval.

We all use time to know when to be somewhere, or when our favorite TV show comes on. In order to know when to leave our current location to arrive at an event "on time", we have to know how long it takes to get there. If you dive, then we use time to tell us our speed (Miles per hour). We use other time measures to know the day of the week, month of the year and even what year it is. Scientist measure time from the very smallest intervals (nanoseconds) to very long intervals (Millennium). This page touches on a few of these concepts but it mainly focuses on the concepts such as "Time Zones" and "Greenwich Mean Time".

   Page Index
 ◊ Greenwich Mean Time (GMT)
 ◊ Time Zones
 ◊ Time Zones - Zulu Time
 ◊ Daylight Saving Time (DST)
 ◊ History behind Time Zones and Greenwich Mean Time
 ◊ Calendar
    ◊ Gregorian calendar
    ◊ Mayan Calendar
    ◊ Other Calendar
 ◊ Definitions
 ◊ Further Reading

Greenwich Mean Time (GMT)

Greenwich Mean Time (GMT) is a term that originally referred to the time at the Royal Observatory, Greenwich, England. Greenwich Mean Time (GMT) is also known as Zulu Time and Coordinated Universal Time (UTC). By definition, the time zone that Greenwich is located in is defined at Zero. In this role as the zero-point reference, Time zones around the world are expressed as positive or negative offsets from GMT (or UTC) normally designated as GMT +/- the time zone. UTC is often referred to as Greenwich Mean Time when describing time zones, although strictly speaking UTC's atomic time scale is only approximately the same as GMT.

In October of 1884, when the International Meridian Conference met in Washington D.C., 25 countries voted to make Greenwich the Prime Meridian of the World. Only the French continued to recognize their own Paris Observatory meridian as 0 degrees until 1911 and even then still could not bring themselves to refer to 'Greenwich Mean Time' but to Paris Mean Time retarded by nine minutes twenty one seconds!

Time Zones

Since there are 24 hours in a day and the Earth is divided into 360 degrees of Longitude, the sun moves across the sky at a rate of 15 degrees of Longitude per hour (360° / 24 hours = 15° / hour). Baku, Azerbaijan is located in time zone plus 4, or GMT + 4. This means that when it is 4:00 pm in Baku, it is 12:00 noon in Greenwich England. The time zones are defined politically, so their centers and boundaries often do not lie on meridians at multiples of 15°. See the "Wikipedia" articles on Universal Time and on the Equation of time for more details.

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Time Zones - Zulu Time

Time Zones A-ZThe world is divided into 24 integer (i.e. 1, 2 3 etc.) time zones. The Time Zones range from -12 through 0 and on to + 12. The International Date Line divides time zone ±12 into two sections (one of - 12; International Date Line west, the other is + 12; International Date Line east). While each time zone has a civilian designations which is typically three letter abbreviations, it also has a military designation. Each time zone is designated by a letter (except "J") and is known by its phonetic equivalent. Greenwich Mean Time is time zone Z or phonically Zulu. The military and aviation normally refer to time by its phonetic equivalent. If you ever watched the television show JAG you may have noticed that the times where shown as Zulu hours.

Example - for a complete list see Military & Civilian Time Designations, From A - Z.
GMT Zone Military Civilian Time Zones Cities
GMT +/- 0 Z Zulu GMT: Greenwich Mean
UT: Universal
UTC: Coordinated Universal
WET: Western European
London, England
Dublin, Ireland
Edinburgh, Scotland
Lisbon, Portugal
Reykjavik, Iceland
Casablanca, Morocco
GMT+1 A Alpha CET: Central European Paris, France
Berlin, Germany
Amsterdam, The Netherlands
Brussels, Belgium
Vienna, Austria
Madrid, Spain
Rome, Italy
Bern, Switzerland
Stockholm, Sweden
Oslo, Norway
GMT+2 B Bravo EET: Eastern European Athens, Greece
Helsinki, Finland
Istanbul, Turkey
Jerusalem, Israel
Harare, Zimbabwe
GMT-5 R   Romeo EST: Eastern Standard. Bogotá, Colombia
Lima, Peru
New York, NY, USA
Roswell, GA, USA
  GMT-6 S   Sierra CST: Central Standard Gadsden, AL, USA
Wichita Falls, Texas, USA
Mexico City, Mexico
Saskatchewan, Canada
  GMT-7 T   Tango MST: Mountain Standard. Parker, AZ, USA
Denver, Colorado, USA
  GMT-8 U  Uniform PST: Pacific Standard Riverside, CA, USA
Los Angeles, CA, USA

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Daylight Saving Time (DST)

Spring forward, Fall back

In 1784 at the age of 78, in a moment of whimsy, Benjamin Franklin wrote An Economical Project, a discourse on the thrift of natural versus artificial lighting. He included several funny regulations that Paris might adopt to help. Over two centuries later, nations around the world use a variation of his concept to conserve energy and more fully enjoy the benefits of daylight. Franklin did not mention Daylight Saving Time and he did not propose that clock time be changed.

The rationale for Daylight Saving Time (DST) is to save energy. The first use of DST was during World War I. It was first adopted by Germany and then later by England and the United States. When the U.S. went on extended DST in 1974 and 1975 in response to the 1973 energy crisis, studies by the US Department of Transportation (DOT) found that observing DST in March and April saved 10,000 barrels of oil a day, and prevented about 2,000 traffic injuries and 50 fatalities saving about U.S. $28 million in traffic costs.

Europeans commonly refer to the system as "summer time", Irish Summer Time, British Summer Time, and European Summer Time. This is reflected in the time zones' names as well, e.g., Central European Time (CET) becomes Central European Summer Time (CEST).

In the Southern Hemisphere DST is observed during their summer months which are from December 21 to June 21. This is just the opposite of the summer months in the Northern Hemisphere. In 2007, DST begins on Sunday, October 28 and will end on Sunday, March 30, 2008, in Adelaide, South Australia (time zone GMT+9:30 hours). Refer to Australia Time Zones for more information.

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History behind Time Zones and Greenwich Mean Time

Why Greenwich Mean Time

Greenwich Mean Time (GMT) was established in 1675, when the Royal Observatory was built, as an aid to determine longitude by mariners at sea. During the 19th century, the British Empire was one the most advanced maritime nations. British navigators as well as mariners from other nations used Nevil Maskelyne´s method of lunar distances based on observations at Greenwich England to determine their Latitude.

On October 22nd, 1707, when a British fleet led by Sir Cloudesley Shovell struck the rocks of Gilstone Ledges on the Scilly Isles, about 2000 men were lost in the accident. This tragedy led the British Parliament to authorize the British Longitude Act in 1714. The British Parliament then authorized the British Longitude Prize for anyone who could devise a practical method of determining longitude at sea. The prize was won by John Harrison in 1759 with the development of the H4 chronometers (watch).

As the British Empire grew in size, England became an advanced maritime nation and the work done at the Royal Observatory in Greenwich England grew in importance. To navigate around the world, mariners kept a chronometer (timepiece) set to Greenwich Mean Time (GMT) in order to calculate their longitude "from the Greenwich meridian", which was, by convention, considered to be zero degrees longitude. This did not affect shipboard time itself, which was still solar time. Knowing the time was so important that ships normally carried three chronometers. One set to local time, one set to GMT and the third as a spare in case one of the other two Chronometers failed.

Greenwich Time Ball

Since accurate timekeeping was so important in enabling mariners to determine their longitude at sea, inventor Robert Wauchope, a Captain in the Royal Navy, developed the time ball. The first time ball was erected at Portsmouth in 1829. Other time balls followed in the major ports of the United Kingdom and around the maritime world. In 1833, Astronomer Royal John Pond installed a bright red Time Ball on top of Flamsteed House at the Greenwich Observatory. This public time signal, distributed time to ships on the Thames and many Londoners, and has dropped at 1pm every day since then.

In 1884, at the International Meridian Conference held in Washington, DC, a proposal was passed to adopt a standard of a universal day of 24 hours beginning at Greenwich midnight (resolution 5). The resolution was passed by a vote of ayes, 15; noes, 2; abstaining, 7.

Adoption of a standard time in England

Before the adoption of standard time zones in the mid 1800s, each town set its clocks based on local noon, or when the sun reached its highest point in the sky. This worked fine when the fastest means of travel was your own two feet or with the aid of an animal such as a horse or an ox. With the development of the railroads it became more important to have some system of standardized time.

The Stockton & Darlington Railroad Company became the first railroad to carry both goods and passengers on regular schedules in September of 1825. Stockton & Darlington used locomotives designed by English inventor, George Stephenson. By 1848 there were a total of 5,000 miles of railway in England. With the expansion of rail service, the Railway Clearing House adopted Greenwich Mean Time as the standard time across all of England in 1847, and by almost all railway companies by 1848. GMT was legally adopted throughout Great Britain in 1880.

Time signals were first broadcast by radio from Greenwich Observatory on 5 February 1924. Sine the daily rotation of the Earth is somewhat irregular and is slowing down slightly, Atomic clocks constitute a much more stable time base. Coordinated Universal Time (UTC) became the international time reference on 1 January 1972. Coordinated Universal Time is maintained by an ensemble of atomic clocks around the world. Leap seconds are added to or subtracted from UTC to keep it within 0.9 seconds of UT1.

Time Zones in the US

US Naval ObservatoryIn 1845, at the request of the Secretary of the Navy, John Y. Mason, the US Naval Observatory (USNO) installed a time ball atop the 9.6-inch telescope dome. The time ball was dropped every day precisely at Noon, enabling the inhabitants of Washington, DC, to set their timepieces and for ships in the Potomac River to set their clocks before putting to sea.

Cleveland Abbe, also known as "The Weather Doctor" recognized that a time-keeping system that was consistent between weather stations was necessary to predict the weather. He divided the United States into four standard time zones. Since Britain had already adopted its own standard time system (based on Greenwich Mean Time) for England, Scotland, and Wales, this helped gather international consent for global time zones. On February 9th, 1870 by a joint congressional resolution signed by President Ulysses S. Grant, the National Weather Service (NWS) was created in the United Sates.

United States railroad companies began utilizing Canadian civil and railway engineer Sir Sanford Fleming's standard time zones on November 18, 1883. At this time the United States government divided the US into four standard time zones (there were only 38 states in 1883). At noon on November 18, 1883, the master clock at the United States Naval Observatory (USNO) transmitted the time by telegraph lines to major cities, each of which adjusted their clocks to their time zone's correct time. The railroads then began using the standard time zones for their schedules.

Beginning in 1904, the U.S. Navy began broadcasting daily time signals by radio. As more countries switched to radio time signals, time balls were replaced. The Royal Greenwich Observatory began broadcasting the time signal on February 5, 1924. As the time balls became obsolete they were demolished. The time ball atop Flamsteed House in Greenwichm England still operates today,

On March 19, 1910, the Standard Time Act established standard time zones in the United States. The Standard Time Act also established daylight saving time (DST). The daylight saving time was repealed in 1919 but was reestablished nationally during World War II. The Uniform Time Act of 1966, signed by President Lyndon Johnson, provided standardization of the dates when DST begins and ends each year but allowed for local exemptions from its observance.

International standard time adopted

The system of international standard time, still in use today, was proposed at the 1884 International Meridian Conference in Washington, DC. Sir Sanford Fleming was instrumental in convening the 1884 International Prime Meridian Conference in Washington, DC. The proposal to adopt a system of standard time by all nations was considered to be outside the purview of the conference, so the proposal was not subjected to a vote. The conference did adopt a universal day of 24 hours beginning at Greenwich midnight and most major countries had adopted hourly time zones by 1929.

Time Ball Trivia
  • Today there are over 60 timeballs standing, including those at the Greenwich Observatory, Edinburgh, Victoria & Alfred waterfront, Point Gellibrand, USA.
  • A modern version of a time ball is the ball drop at the stroke of midnight on December 31, notably in Times Square, New York City and the Peach Drop in Atlanta, Georgia.
  • As part of the Millennium Program , the USNO Time Ball was dropped at midnight EST on New Year's Eve 2000 to usher in the year Millennium.

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A calendar is a system of organizing units of time for the purpose of tracking time over extended periods. The smallest unit of time used on a calendar is a day, or one revolution of the Earth. Dividing a day into smaller segments of time (hours, minutes, and seconds) is classified as timekeeping.

It is estimated that there are about different 40 Calendars currently in use around the world (Fraser, 1987).

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Gregorian calendar

Today, most modern countries use the Gregorian calendar for their official activities. The Gregorian calendar is based on the Julian calendar which was introduced by Julius Caesar in 45 BC. The Julian calendar consisted of eleven months of 30 or 31 days plus a 28-day February (extended to 29 days every fourth year). The Julian calendar has 1 leap year every 4 years.

The Julian calendar was quite accurate, but reform was necessary because too many leap days are added with respect to the astronomical seasons. It differed from the solar calendar by only 11½ minutes a year, but over many centuries even this small inaccuracy added up. The Julian calendar introduces an error of 1 day every 128 years. So every 128 years the tropical year shifts one day backwards with respect to the calendar. By 1582, the Julian calendar was 10 days behind the solar calendar.

To correct this difference, in 1582, Pope Gregory XIII decreed that the day after October 4, 1582 would be October 15, 1582. Because it was Pope Gregory's that decreed this change, the reformed calendar became known as the Gregorian calendar.

The Gregorian calendar is divided into 12 months. Each month has either 30 or 31 days except February which has just 28 days. Since this arrangement only equals 365 days and there are 365 and 1/4 days in a year, every four years one day is added to the month of February to keep the Calendar synchronized with the astronomical or seasonal year. In addition, if the last two digits of the year are zero (1800, 1900, 2000), then only if the year is evenly divisible by 400 will a day be added to the month of February. This is known as a leap year.

Month Number if days
February28 or 29
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Mayan calendar

The Mayan civilization, of southern Mexico, Belize, Guatemala, El Salvador, and Honduras, developed a written language and a complex and remarkably accuracate Calendar. The Maya calendar uses three different dating systems in parallel, the Long Count, the Tzolk´in (a 260 day divine calendar), and the Haab´ (a 365 day civil calendar). Of these, only the Haab´ has a direct relationship to the length of the solar year.

The Tzolk´in, or ritual calendar, used a count of 260 days. This calendar gave each day a name, much like our days of the week. There were 20 day names, each represented by a unique symbol. The days were numbered from 1 to 13. Since there are 20 day names, after the count of thirteen was reached, the next day was numbered 1 again.

The day names were:
0. Ahau 1. Imix 2. Ik´ 3. Ak´bal 4. K´an
5. Chicchan 6. Cimi 7. Manik´ 8. Lamat 9. Muluc
10. Oc 11. Chuen 12. Eb 13. Ben 14. Ix
15. Men 16. Cib 17. Caban 18. Etz´nab 19. Caunac

Haab´, or civil calendar, had 365 days and is comprised of eighteen "months" of twenty days each. Eighteen "months" of twenty days adds up to 360 days. A period of five nameless days occurred at the end of the year and is known as Wayeb´.

The names of the month were:
1. Pop 7. Yaxk´in13. Mac
2. Uo 8. Mol 14. K´ank´in
3. Zip 9. Ch´en 15. Muan
4. Sotz´10. Yax 16. Pax
5. Tzec11. Zac 17. K´ayab
6. Xul 12. Ceh 18. Kumk´u

Haab´ month names changed every 20 days instead of daily; so the day after 1 Ceh would be 2 Ceh, followed by 3 Ceh ... up to 19 Ceh, which is followed by 0 Mac.

The year is not numbered in the Tzolk´in nor the Haab´ calendar systems. The combination of a Tzolk´in date and a Haab´ only occurs once every 52 years so this was enough to identify a date to most people´s satisfaction. Such a combination would not occur again for another 52 years, above general life expectancy. The Maya usually described a date by specifying its position in both the Tzolkin and the Haab calendars, this alignment of the Sacred Round and the Haab calendar (also knowen as the Vague Year) generates the joint cycle called the Calendar Round.

Mayan Calendar Round
1/2/2007 -  8 Ahau 13 K´ank´in
1/3/2007 -  9 Imix 14 K´ank´in
1/4/2007 - 10 Ik   15 K´ank´in

The Long Count represents the number of days since the start of the Mayan era. It is really a mixed base-20/base-18. The first day of the long count of the current Mayan era is written as The current Mayan era began on September 18, 1618. The "long count" is a continuous record of days from a zero date that correlates to Aug. 13, 3114 BC,

Wikipedia gives the following information and conversion.
The Mayan name for a day was k'in; twenty of these k'ins are known as a winal (or uinal); eighteen winals make one tun; twenty tuns are known as a k'atun, twenty k'atuns make a b'ak'tun.

Table of Long Count units
Days Long Count periods Long Count Solar years Tuns
1 = 1 K'in      
20 = 20 K'in = 1 Winal    
360 = 18 Winal = 1 Tun ~ 1 1
7 200 = 20 Tun = 1 K'atun ~ 20 20
144 000 = 20 K'atun = 1 B'ak'tun ~ 395 400

Friday February 9, 2007 at 22:16 (UTC), in the Long Count is

The long count for some other dates courtesy of MichielB´s Project are as follows:

A note on the spelling of Myan terms.

While I was gathering information on the Mayan Calendar, I noticed that the spelling of the Mayan terms was not consistent from web site to web site and sometimes it was not even consistent on the same web site. I have tried to be consistent on this web page, but I do not know if I have spelled the Mayan terms correctly or not.

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Other calendars from around the world

Most modern countries use the Gregorian calendar for their official activities, but there are several other calendars still in use. An example is People´s Republic of China. China uses the Gregorian calendar for civil purposes as well as a special Chinese calendar for determining festivals. Below is a partial list of some other types of calendars.

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A lightyear is a measure of distance; not of time.

A lightyear is the distance light travels in a vacuum in one Julian year. Light travels at the speed of 299,792 kilometers per second. That is 186,282 miles per second. With 31,557,600 seconds in a year, one lightyear equals a distance of 5.87 trillion miles or 9.46 trillion kilometers.


A year is how long it takes for a planet to complete one Orbit around its sun. The Earth takes 365 days 5 hours 48 minutes to complete one orbit.

Leap Year

A leap year is a year containing an extra day in order to keep the calendar year synchronized with the astronomical or seasonal year. During a leap year we add one day to the month of February, so February has 29 days rather than 28.

How to calculate Leap Year.
If the last two digits of the year are zero (1800, 1900, 2000), then divide the year by 400.
If the last two digits are not zero (2001, 2002, 2004), then divide the year by 4.
If the result of the division is a whole number, then it is a leap year.

Year Divide By Results Leap Year?
1800 400 4.5 No
1854 4 463.5 No
1856 4 464 Yes
1900 400 4.75 No
1950 4 487.5 No
1952 4 488 Yes
1994 4 498.5 No
1996 4 499 Yes
1999 4 499.75 No
2000 400 5 Yes
2001 4 500.25 No
2002 4 500.5 No
2003 4 500.75 No
2004 4 501 Yes
2008 4 502 Yes

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A day is the length of time it takes for a planet to complete a single rotation with respect to its Sun. The planet Earth takes 23 hours and 56 minutes to compete a single rotation.

A day is commonly divided into 24 hours of 60 minutes, each with 60 seconds. The present common convention has the day starting at midnight.


A minute is a unit of time equal to 1/60th of an hour or 1/1,440 day. A minute also equals 60 seconds.


A second is a basic unit for the measurement of time. It is defined under the International System of Units (SI), as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition refers to a cesium atom at rest at a temperature of 0 K (absolute zero).

  • 1/60 minute (1 minute is equal to 60 seconds)
  • 1/3,600 hour (1 hour is equal to 3,600 seconds)
  • 1/86,400 day (1 day, in the sense of non-SI units accepted for use with the International System of Units, is equal to 86,400 seconds)
  • 1/31,557,600 Julian year (1 year, in the sense of non-SI units accepted for use with the International System of Units, is equal to 31,557,600 seconds)

The international standard symbol for a second is s (see ISO 31-1).

Leap second

Leap-seconds are one-second adjustments used to keep the broadcast standard time of day close to mean solar time. This keeps the broadcast time standard which is maintained using extremely precise atomic clocks synchronized with civil calendars. Cival Calendars are based on astronomical events.

Nanosecond (1 E-9 s )

A nanosecond is one billionth of a second; (10–9 second).

Grace Hopper is famous for handing out pieces of wire which were just under one foot long (11.8 inch), which is the distance that light travels in one nanosecond, which she used to help explain timing of satellite communication and computers.

Femtosecond (1 E-15 s )

A femtosecond is one billionth of one millionth of a second; (10–15 second). For context, a femtosecond is to a second, what a second is to a hundred million years.

Attosecond (1 E-18 s )

An attosecond is one billionth of one billionth of a second; (10–18 second). One hundred attosecond is to one second what one second is to 300 million years.

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Further Reading:

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  1. ^ Tim Folger "In No Time" Discover.  June 2007:79
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