But what if you need to know how many seconds there will be between now and a year from now? If you think that you can just multiply out the result, you'd be wrong. A year isn't a fixed number of seconds, it's the time it takes the Earth to make one full revolution around the sun. And, depending on how you look at it, a day doesn't have 24 hours either. If a day is the amount of time it takes the Earth to make one full rotation, then a "day" is almost exactly 23 hours and 56 minutes. But since the Earth revolves around the sun while it rotates about its axis, the sun or a star will appear to cross the meridian after approximately 24 hours. But the Earth's orbit isn't circular, and the equatorial plane is inclined to the orbital plane. Thus a "day," measured by the sun crossing the meridian, varies in length throughout the year. To make things worse, the sun isn't standing still either, so the length of a year changes over time.

Let's Get Technical
Sidereal time is the hour angle of the vernal equinox, the ascending node of the ecliptic on the celestial equator. The daily motion of this point provides a measure of the rotation of the Earth with respect to the stars, rather than the Sun. Local mean sidereal time is computed from the current Greenwich Mean Sidereal Time plus an input offset in longitude (converted to a sidereal offset by the ratio 1.00273790935 of the mean solar day to the mean sidereal day). Applying the equation of equinoxes, or nutation of the mean pole of the Earth from mean to true position, yields local apparent sidereal time. Astronomers use local sidereal time because it corresponds to the coordinate right ascension of a celestial body that is presently on the local meridian.

International Standard Time Terms

• Mean Solar Day
  24 hours. An average value used for convenience. Usually corresponds to 86,400 standard seconds.

• Standard Second
  A relatively invariant amount of time. There are usually 60 standard seconds to the standard minute, 60 standard minutes to the standard hour, and 24 standard hours to the standard day. The length of a standard second is derived from the decay time of cesium atoms. It is close to, but not identical with, the length of time measured by a solar second. By international agreement, a standard second is "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 cesium atom." (Did you really want to know that?)

• Leap Seconds
  Because the standard second is stable, while the motion of the Earth is not, the standard minute is occasionally adjusted by adding a leap second. Some minutes, therefore, have 61 seconds. Standard hours always have 60 minutes, even if one of the minutes is a second longer than usual. Standard days always have 24 hours.

• Greenwich Mean Time (GMT)
  GMT is a mathematical mean, defined in terms of the solar second, measured at the former location of the Royal Observatory in England, located on the Prime Meridian (zero degrees longitude). GMT is useful for navigation (when converted to UT1, which is outside the scope of this document), but not for time-keeping programs.

  When speaking loosely, GMT and Coordinated Universal Time (UTC) are roughly the same, but careful speakers will note that GMT is based on the solar second, while UTC is based on the standard second. The delta between the two time systems is too small for humans to notice, but very important for computer programs and universal synchronization.

• Coordinated Universal Time (UTC)
  There are seven "universal" times, all currently within one second of each other, used for various purposes. The Coordinated Universal Time is the one used for timesetting. The abbreviation UTC is a language-independent international abbreviation. It means both "Coordinated Universal Time" and "Temps Universel Coordonné." (If it strikes you as odd that neither of these two word orders can be abbreviated to UTC, you're not alone. For more information, we refer you to http://www.bipm.fr/enus/5_Scientific/c_time/time_1.html.)

  UTC is the time-zone-independent reference standard used by time-keeping programs. UTC is based on the standard second, so varies occasionally from GMT, but is corrected by the periodic application of a leap second.

  Domain Time uses UTC internally when serving and obtaining the time. Conversion to local time (i.e., the time as defined by your time zone and current daylight-savings corrections) is performed for display purposes only. Thus, any Domain Time Server anywhere in the world can provide the time to any Domain Time Client. Each machine will display local time, but communicate using UTC. Time zones and daylight-savings time do not affect Domain Time's accuracy.

PC Timekeeping Terms

• Accuracy
  The degree of conformity of a measured or calculated value to its definition, or with respect to a standard reference. Also the closeness of a measurement to the true value as fixed by a universally accepted standard. For PC timekeeping, accuracy usually means "how closely this clock matches the reference clock" within the ability to measure the difference.

• Precision
  The degree of mutual agreement among a series of individual measurements. Precision is often, but not necessarily, expressed by the standard deviation of the measurements. Also, random uncertainty of a measured value, expressed by the standard deviation or by a multiple of a standard deviation. For PC timekeeping, precision refers to how well a particular machine keeps the time, determined by the short-term fluctuations in frequency of the computer clock's oscillator, and by measurement errors introduced by the overall clock resolution, interrupt latencies, preemption latencies, and processor load. Although precision has a precise technical definition, the word is often used in casual speech as if it were synonymous with accuracy.

• Resolution
  The degree to which a measurement can be determined is called the resolution of the measurement, i.e., the smallest significant difference that can be measured with a given instrument. For example, a measurement made with a time interval counter might have a resolution of 10 nanoseconds; periods of time smaller than the resolution are imperceivable to the instrument doing the measuring.

• Monotonicity
  If a clock has monotonicity, then each successive time reading from that clock will yield a time further in the future than the previous reading. Rapid successive checks may yield the same time as the last reading if the check interval is smaller than the resolution of the clock or the test instrument. Monotonicity is very important for many tasks, such as elapsed time measurement, time-date stamps on documents or events, system logs, and so forth. Precision clocks and hardware oscillators almost always display monotonicity due to their physical nature of incrementing a counter at a fixed interval, but software clocks are under no such restriction. Good PC timekeeping software restricts backward-travelling clocks as much as possible within the constraints set by the machine's administrator.

Time Measurement Terms

• Millisecond
  One-thousandth of a second (0.001). Typically the best accuracy that can be achieved on a PC.

• Microsecond
  One millionth of a second (0.000001). Used primarily by lab equipment, GPS receivers, or other hardware.

• Hectonanosecond
  100 nanoseconds (0.0000001), or 1/10th of a microsecond. The base internal unit used by Windows, NT, and 2000 machines to track the passage of time. Windows calculates elapsed time as x hectonanoseconds per clock interrupt. The value of x varies depending on the machine's hardware, operating system, and speed.

• Nanosecond
  One billionth of a second (0.000000001). Used primarily by physicists and electronics engineers in measurements of very small intervals, such as RAM access times, or the time it takes light to cross a room.

• PC Timer Resolution
  Approximately 0.0555 seconds, or 1/18th of a second. The interval between hardware interrupts on a standard PC. This value represents the maximum accuracy that can be obtained when setting or reading the clock under Windows 95, Windows 98, or Windows ME.

• Kilosecond
  1000 seconds, or approximately 16 minutes. The amount of time it takes the average person to read this page.