Theory 
Time-Keeping: Time| CONTENT - How time has been calculated along the ages and is now |
The daily rotation of Earth -bringing sunrise in the morning and sunset in the evening- obviously was the natural clock to mankind since the highest antiquity. The refinement of timekeeping systems, then the apparition of clocks, together with the development of natural sciences improved timekeeping and the measure of Earth's rotation
It's not before the middle of the Middle Ages that the first clocks appeared. Until then time was kept with various devices allowing to get an estimate of what hour or what part of the day it was, or of how much time was elapsing. Sundials (some of them large, like the obelisks of the Egyptian temples) or clepsydras (water clocks), candles marked in increments or hourglasses were used. First clocks eventually appeared about 1350, in Italy. Clocks remained heavy, then spring-driven until the Dutch scientist Christiaan Huygens managed to built the first pendulum clock in 1656. Its clock managed to indicate time with an error of less than 10 seconds a day. Clocks and science was working hand in hand
Clocks continued to improve, mainly for the use of the English navy. Longitude needed to be accurately measure to avoid wreckages and it could not be except with very accurate time-keeping systems. The Astronomer Royal and Greenwich, beginning in 1675, then clock-makers, made progresses for such a purpose. By 1761 Harrison's marine chronometer was able to determine longitude to within half a degree after a journey to the West Indies, keeping time with an accuracy of one-fifth of a second a day. Pendulum clocks were further improved by freeing the pendulum of any interference as by the 1920's the next step was the quartz-based clocks. An electric field is applied to a quartz crystal; the crystal changes its shape, generating in turn an electric current. This brings to a vibration and an electric signal which are used for the clock machinery. Eventually scientists deviced atomic clocks where an electromagnetic wave interacting with an atom generates accurate and steady signals
As tools were progressing, the time remained kept at a local level only. Each city hall, each town, worked on the local solar time. As soon as the new transportation means developed -namely the railroads- allowing swift deplacements in one country, this disparity in time-keeping became an obstacle. It was hard to maintain train schedules as time was varying, inside a same country, along a line East-West! Progressively one came to unify time systems inside a same country and, eventually, the International Meridian Conference in Washington in October 1884 set up and international, uniformized, time system. An internationally enforced meridian system was put into place with the meridian of the Observatory of Greenwich like the initial meridian. All longitudes became calculated East or West of it up to 180°. At the same time all countries were adopting a universal time system based on the mean solar time. A mean solar day of 24 hours was adopted as 24 timezones were deviced. Each timezone was centered on a standard meridian giving it its mean solar time. The 24 timezones were set relative to the mean solar time of Greenwich (Greenwich Mean Time, GMT). They divided the world into theoretically 15° wide zones heading east or west of Greenwich. Where they meet, at longitude 180° the time zone is parted into two parts, with the International Date Line running. When a traveler passed the line heading East it came back to the previous day. Heading West it came to the next day. The GMT system was at the same time a timezone and a time, strictly, system. The idea of meridian had been set up for the first time in the USA as, when the first transcontinental railroad had been completed by May 1869, they needed to establish a regular timetable
The GMT system was transformed about 1926 into the Universal Time (UT) system. This passage merely was a change of terms as the system described previously remained enforced, and still is today. Time UT came too to be used too as the basic astronomical time. After WW2, the progress of time-keeping and the need for highly accurate clocks, brought the atomic clocks, which are working based on the frequency of electromagnetic waves emitted or absorbed by an atom of Cesium-133. The international time-keeping institutions eventually came to these clocks as the new base for the civil time worlwide. Such clocks are working on the basis of an internationally defined second, the SI-second, which is the second of the International System of units. It's based on such a second that the Coordinated Universal Time (UTC) became the time used in your watch and at time services. Time UT was maintained as the time for the timezones system as UTC became the civil time. Time UTC is not an astronomically-defined time anymore however. It's a theoretical time, defined by the atomic clocks, as it is calculated based on readings from more than 200 atomic clocks maintained across the world. It's the International Bureau of Weights and Measures outside Paris, which has in charge the atomic time. Due to Earth's rotation irregularities -and the tendency of it to slow due to Moon interaction (roughly every 100 years, the day gets about 1.4 milliseconds longer)- this accurate atomic time does not match exactly the Earth's rotation rate. To avoid any discrepancy, a positive, or negative, "leap second" is added to, or substracted from, the atomic clocks when needed. The leap second may be added on Dec. 31, as sometimes it's inserted by the end of June 30. This way, the time UTC is kept within 0.9 second from time UT (which is a time with an astronomical reference -for more details see the tutorial Time Systems in the Observation section, Tutorials). The discrepancy also comes from the fact that deterioration of the Cesium atom cannot keep up with the rotation of the Earth. The current time-keeping system is operated at the U.S. Naval Observatory, in Washington, D.C. as Canada, France, the UK, and European CERN which had duplicated the system at the National Research Council of Canada at Ottawa, the CEA Center of Saclay (France), Herstmonceux Castle, Hailsham (UK), and the CERN (European Center for Nuclear Research) physics of particle laboratory in Neuchâtel (Switzerland) respectively, are no longer willing to support their own laboratories and now depend upon time signals from Washington. The leap second, formally, is decided by the 'International Earth Rotation and Reference Systems Service' (IERRSS). Leap second was introduced in 1972 and IERRSS last addition was on December 2008 as a next one is scheduled June 2012. Proposals have been made to abolish the leap second due to the cost of adjusting or turning important systems on and off in synch. By 2015 however, the International Telecommunication Union (ITU), a specialized agency of the United Nations, announced that it would defer further a decision until 2023, when it will have more information on the impacts of getting rid of the leap second. The union did, however, made a decision that could shift the responsibility of defining the official Coordinated Universal Time (UTC) -and in turn the leap second- to the General Conference on Weights and Measures (CGPM), which already has ultimate responsibility for defining SI units. BIPM, a subsidiary of the CGPM, is responsible for generating International Atomic Time, on which UTC is based, from the results of 500 clocks distributed around the world. Most countries, including China, the United States and many in Europe, favour scrapping the leap second and basing UTC on the continuous tick of atomic clocks only, as a small number of countries however, including Russia and the United Kingdom, want to keep the leap second. Russia is concerned about how GLONASS, its Global Navigation System which is the only one to incorporate leap seconds. Should no more a leap second be used, that would translate into a 25-minutes discrepancy in 500 years from now. Inserting the leap second can disrupt computer networks and thus need to provide a six-month notice. The last minute of the year 2016 will recognize a leap second as the last one had occurred on June 30, 2015. The atomic clocks gave birth to another time system, which is used for highly accurate science works only. It's the Atomic Time (TAI, from the French "Temps Atomique International"). As far as the GPS (Global Positioning System) time aspect is concerned, the clocks both on Earth devices and aboard satellites (these are atomic clocks) started with an offset of 19 seconds compared to UTC. This was due to the GPS beginning to work on January 6th, 1980 when such an offset existing between time TAI and UTC. GPS receivers simply translate this offset, plus the current offset, when they receive the satellite data, and they display the time UTC. for more technical details about these timekeeping systems and some others more especially dedicated to astronomy, see at the tutorial "Time Systems" in the Observation section
->The Atomic Time To Overpower the GMT in 2009?
The difference between the GMT way of measuring time, and the time computed by atom clocks is of such an amount, on large scales of time, that, within a millenium, noon would occur about 1 p.m. as, in tens of thousands of years, the Sun would be days behind the regular calendar! Some thus advocate that the GMT time system should be definitively replaced by the atomic one due to that the addition of the leap second can trip up time-sensitive softwares. Britons, on the other hand, as far as they pride about Greenwich is concerned, are concerned that the atom-measured time is of the resort of France, at the International Bureau of Weights and Measures outside Paris. France, along with Germany, Italy, the U.S., and Japan, are supporting the end of the leap second system, as the International Telecommunications Union is to vote on a switch as early as in 2009, with a 2018 target to implement it. The main argument of the GMT and the leap second proponents is that to keep the GMT system is in a better accordance with the position of the Sun in the sky. A switch from the Sun-based to the atom-based data would represent, in any case, a cultural shift of sort
