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Frequently Asked Questions About Time and Timekeeping

"Do not squander time, for that's the stuff life is made of." - Benjamin Franklin


Time and Time Standards
    Local Time
    Universal Time
    Coordinated Universal Time (UTC)
    Greenwich Mean Time
    NIST Time Servers
    New NIST Cesium Atomic Clock is World's Most Precise
    NIST Time Setting Format
The World is Slowing Down
More on the Leap Second
Origins of Daylight Savings Time in the U.S.


Related links:

FAQ for PC clocks
GPS FAQ    GPS technology
History of timekeeping
ClockWatch main page


Time and Time Standards

Local Time The Time shown here (with a Java enabled browser) is the System time from the local operating system. This time is based on Universal Time offset for the local time zone and Daylight Saving Time.

Universal Time (UT).  The Universal Time Family is the general designation of time scales based on the rotation of the Earth. In applications in which a precision of a few tenths of a second cannot be tolerated, it is necessary to specify the form of UT such as UT1 which is directly related to polar motion and is proportional to the rotation of the Earth in space. The UT1 is further corrected empirically for annual and semiannual variations in the rotation rate of the Earth to obtain UT2.

Universal Time is the mean solar time of the prime meridian plus 12 hours, determined by measuring the angular position of the Earth about its axis. The UT is sometimes designated Greenwich Mean Time (GMT), but this designation should be avoided. Communicators use the designation (Z) or (Zulu). Timekeepers should use UTC for the national standard - for example, UTC (USNO) rather than GMT.

Mean Solar Time is simply apparent solar time corrected for the effects of orbital eccentricity and the tilt of the Earth's axis relative to the ecliptic plane; that is, corrected by the equation of time which is defined as the hour angle of the true Sun minus the hour angle of the mean Sun.

Coordinated Universal Time  (UTC) is a coordinated time scale maintained by the Bureau International des Poids et Mesures (BIPM), which forms the basis of a coordinated dissemination of standard frequencies and time signals. NOTE: A UTC clock has the same rate as a Temps Atomique International (TAI) clock or international atomic time clock, but differs by an integral number of seconds called leap seconds. The UTC scale is adjusted by the insertion or deletion of seconds (positive or negative leap seconds) to ensure approximate agreement with UT1 (also known as the Julian Date)

Greenwich Mean Time (GMT) is a 24 hour astronomical time system based on the local time at Greenwich, England. GMT can be considered equivalent to Coordinated Universal Time (UTC) when fractions of a second are not important. However, by international agreement, the term UTC is recommended for all general timekeeping applications, and use of the term GMT is discouraged.

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Does anyone really know what time it is? Well, the U.S. Government wants to, so they created the National Institute of Standards and Technology, a component of the U.S. Department of Commerce. The Time and Frequency Division, located in Boulder, Colorado, maintains the F-1 Fountain Atomic Clock, the nation’s standard of time. This clock neither gains nor loses a second over a one million year period. This clock is used to create an international time scale, which NIST distributes through its time servers

NIST Time Servers

The Time and Frequency Division is an operating unit of the Physics Laboratory of the National Institute of Standards and Technology (NIST). Located in Boulder, Colorado at the NIST Boulder Laboratories, the Time and Frequency Division:

  • Maintains the primary frequency standard for the United States.
  • Develops and operates standards of time and frequency.
  • Coordinates U. S. Time and Frequency standards with other world standards.
  • Provides time and frequency services for United States clientele.

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New NIST Cesium Atomic Clock is World's Most Precise

Termed NIST F-1, the new cesium atomic clock at NIST's Boulder, Colo., laboratories, began its role as the Nation's primary frequency standard by contributing to an international pool of the world's atomic clocks that is used to define Coordinated Universal Time (known as UTC), the official world time. Because NIST F-1 shares the distinction of being the most accurate clock in the world (with a similar device in Paris), it is making UTC more accurate than ever before. NIST F-1 recently passed the evaluation tests that demonstrated it is approximately three times more accurate than the atomic clock it replaces, NIST-7, also located at the Boulder facility. NIST-7 has been the primary atomic time standard for the United States since 1993 and is among the best time standards in the world.

NIST F-1 is referred to as a fountain clock because it uses a fountain-like movement of atoms to obtain its improved reckoning of time. First, a gas of cesium atoms is introduced into the clock's vacuum chamber. Six infrared laser beams then are directed at right angles to each other at the center of the chamber. The lasers gently push the cesium atoms together into a ball. In the process of creating this ball, the lasers slow down the movement of the atoms and cool them to near absolute zero.

Two vertical lasers are used to gently toss the ball upward (the "fountain" action), and then all of the lasers are turned off. This little push is just enough to loft the ball about a meter high through a microwave-filled cavity. Under the influence of gravity, the ball then falls back down through the cavity.

As the atoms interact with the microwave signal - depending on the frequency of that signal - their atomic states might or might not be altered. The entire round trip for the ball of atoms takes about a second. At the finish point, another laser is directed at the cesium atoms. Only those whose atomic states are altered by the microwave cavity are induced to emit light (known as fluorescence). The photons (tiny packets of light) emitted in fluorescence are measured by a detector.

This procedure is repeated many times while the microwave energy in the cavity is tuned to different frequencies. Eventually, a microwave frequency is achieved that alters the states of most of the cesium atoms and maximizes their fluorescence. This frequency is the natural resonance frequency for the cesium atom - the characteristic that defines the second and, in turn, makes ultra-precise timekeeping possible.

The NIST F-1 clock's method of resolving time differs greatly from that of its predecessor, NIST-7. That device - and the versions before it - fired heated cesium atoms horizontally through a microwave cavity at high speed. NIST F-1's cooler and slower atoms allow more time for the microwaves to "interrogate" the atoms and determine their characteristic frequency, thus providing a more sharply defined signal.

NIST F-1 was developed by Steve Jefferts and Dawn Meekhof of the Time and Frequency Division of NIST's Physics Laboratory in Boulder, Colo. It was constructed and tested in less than four years.

This new standard is more accurate, by a wide margin, than any other clock in the United States and assures the nation's industry, science and business sectors continued access to the extremely accurate timekeeping necessary for modern technology-based operations. Together with the U.S. Naval Observatory in Washington, D.C., NIST provides official time to the nation.

NIST Boulder Press Release, December 29, 1999

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NIST Time Setting Format

The NIST transmits in its own standard Automated Computer Timer Service (ACTS). It is contacted via TCP/IP on port 13. After a setting is made, the time string from the NIST used in the setting is displayed in the NIST log window. ClockWatch translates this string. All times from NIST are in UTC. This time string is made up of a series of fields arranged end to end.

Message Format received from NIST, with an actual sample string below it:

49010 93-01-23 22:01:22 00 0 0 50.0 UTC(NIST) *

MJD:  The first number is the date expressed as a Modified Julian Day number (MJD);  in the above example 49010 is the Modified Julian Day. The Modified Julian Day  is obtained by counting days from the starting point at midnight on 17 November 1858. It is one way of telling what day it is with the least possible ambiguity.

YYMMDD HHMMSS:  The next 6 values give the Universal Coordinated date and time (formerly called Greenwich Mean Time) as year, month, day, hour, minute and second.

DST:  The eighth number is the daylight saving time flag, DST.  It is based on the continental US system, which has transitions on the first Sunday in April and the last Sunday in October.
DST = 0 means standard time is currently in effect.
DST = 50 means daylight saving time is currently in effect.
DST = 51 means the transition from standard time to daylight time is at 2am local time today.
DST = 1 means the transition from daylight time to standard time is at 2am local time today.
DST > 51 gives advance notice of the number of days to the transition to daylight time. The DST parameter is decremented at 0000 every day during this advance notice period, and the transition will occur when the parameter reaches 51 as discussed above.
1 < DST < 50 gives advance notice of the number of days to the transition to standard time. The DST parameter is decremented at 0000 every day during this advance notice period, and the transition will occur when the parameter reaches 1 as discussed above. The DST parameter is usually not needed for UNIX systems which keep time internally using Universal Time.
Note: ClockWatch uses the Windows internal Time Zone setting to determine if daylight savings time is both used and in effect.

LS:  The next number is the leap second flag, LS.
LS = 0 means no leap second is scheduled.
LS = 1 means that a leap second is to be added as 23:59:60 on the last day of the current month. The last minute will therefore be 61 seconds long. Leap seconds are usually added at the end of either June or December.
LS = 2 means that second 23:59:59 is to be dropped on the last day of the current month. The second following 23:59:58 will be 00:00:00 of the next day. This minute will therefore be 59 seconds long. This situation is unlikely to be necessary in the foreseeable future.
Note: Leap seconds are inserted or deleted at the specified Universal Times, while daylight savings transitions are always with respect to local time.

H:  The health parameter, H, gives the health of the timeserver:
H = 0 means that the server is healthy.
H = 1 means that the server is operating properly but that its time may be in error by up to 5 seconds. This state should change to fully healthy within 10 minutes.
H = 2 means that the server is operating properly but that its time is known to be wrong by more than 5 seconds.
H = 3 means that the hardware or software has failed and that the time error is unknown.

ADV:  The advance parameter, ADV, gives the time advance of the transmissions, in milliseconds. Each time packet is sent out early by this amount to compensate (approximately) for the network delay.

MISC:  The remaining characters on the line identify the time source and are included for compatibility with the ACTS time system.

*Prepared with information obtained from the NIST, Boulder, CO

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The World is Slowing Down

The Earth's official consortium of timekeepers, officials from around the globe, will add a leap second to the world's atomic clocks at the end of the year to keep the official atomic time synchronized with natural time as determined by the spin of our planet. The leap second will be added to more than 60 atomic clocks operating around the world so that we can keep track of time with an accuracy of 10 billionths of a second per day.

The old-fashioned way of measuring time, based on when the sun rises and sets, is good to about one thousandth of a second per day. Scientists try to keep the atomic clocks, which are based on the vibration rates of cesium or hydrogen atoms, within nine-tenths of a second of the Earth's rotation time. Because the Earth is slowing down, scientists have had to add 22 leap seconds since 1972.

Reference: Minneapolis Star Tribune, September 9, 1998

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More on the Leap Second

Good news for all those computer engineers scrambling to reprogram the world's computers before the year 2000 strikes: You'll have one second more time than you thought to get the job done.

Timekeepers at the U.S. Naval Observatory have determined that a "leap second" must be added at the end of this year to synchronize the nation's official timepiece - the observatory’s so-called Master Clock - with actual time as measured by the Earth's rotation.

The Master Clock is a consensus chronometer based on time readings from more than 60 independently operating atomic clocks kept in a dozen sealed vaults. It is accurate within a billionth of a second per day, which is problematic since the Earth's rotation is not nearly as regular as that. In fact, the Earth has been spinning down of late, turning a little slower each year. If a second or two were not added occasionally to the world's atomic clocks, these timekeepers would eventually be saying it's noon even as the sun was just coming up over the eastern horizon.

The adjustment has more immediate significance, however. Many electronic navigation and communication systems - including the global positioning system, which can tell people exactly where they are on the planet - depend on extremely precise measurements of time intervals. A lag of even a few billionths of a second can lead to significant inaccuracies.

The leap second will be added to the world's atomic clocks December 31 at 23:59:59 Greenwich Mean Time (6:59:59 p.m. Eastern Standard Time).

Reference:  Washington Post September 14, 1998

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Origins of Daylight Savings Time in the United States

The United States adopted the Daylight Savings Time plan in 1918, but repealed it in 1991. It also observed daylight time from Feb. 9, 1942 to Sept. 30, 1945, to conserve energy during World War II. After the war, many states established some sort of daylight savings time. Beginning in 1967, nearly all the states went on daylight time from the last Sunday in April to the last Sunday in October.  In the 1970s, during the oil crisis, Congress enacted daylight time from Jan 6 to Oct. 27, 1974, and from Feb. 23 to Oct 26, 1975, again to conserve energy. Since 1987, daylight time has begun on the first Sunday in April.

Reference:  New York Times and Cox News Service May 7, 1999

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Last reviewed September 13, 2004