Universal Time
Based on a given local time, the calculator computes Universal Time (UT1), Coordinated Universal Time (UTC), International Atomic Time (TAI), and Terrestrial Time (TT) using data from the International Earth Rotation and Reference Systems Service. Changes between Universal Time and Coordinated Universal Time can be viewed on a graph.
Based on a specified local time, this calculator displays the current Universal Time (UT1), Coordinated Universal Time (UTC), International Atomic Time (TAI), and Terrestrial Time (TT). It also shows the difference between UT1 and UTC for the given date, as well as the number of leap seconds. The calculator operates using data from the International Earth Rotation and Reference Systems Service (IERS)[^iers]. Data is updated daily, and a forecast for several months ahead is available.
Universal Time
Universal Time (UT)
Universal Time is a general term for time scales based on Earth's rotation. Currently, the UT1 and UTC scales are the most commonly used. The UT1 scale corresponds to local solar time at the longitude of the Greenwich Observatory. UT1 is measured in proportion to Earth's rotation angle relative to the International Celestial Reference System (ICRS) using Very Long Baseline Interferometry (VLBI). Earth's rotation is irregular—exhibiting short-term fluctuations as well as a slight, constant deceleration—so the UT1 scale cannot be used for precise astronomical calculations. Once this fact was discovered, astronomers began using Ephemeris Time for precise calculations; this is a uniform time scale based on Earth's annual orbit around the Sun. Ephemeris Time was first defined in 1952 as 1/31,556,925.975 of the tropical year (the period of Earth's orbit around the Sun as of January 1, 1900). In 1960, the second was defined within the SI system as 1/31,556,925.9747 of the tropical year. ## International Atomic Time (TAI)
By 1955, a more precise method of timekeeping was already known. Consequently, the ephemeris second in the SI system remained in use for only seven years; in 1967, the definition was replaced by atomic time, which could be measured with extreme precision using atomic clocks. The atomic second was set equal to the ephemeris second and defined 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 cesium-133 atom. Timekeeping based on the TAI atomic time scale began in 1958. On January 1, 1958, the TAI value equaled the UT2 value and differed only slightly from UT1 (UT2 is a smoothed version of UT1, free from periodic seasonal fluctuations).
Coordinated Universal Time (UTC)
Over time, the difference between universal time (UT1) and atomic time (TAI) has grown, exceeding 37 seconds as of April 2021. As a compromise between uniform atomic time and the familiar universal time (UT1), Coordinated Universal Time (UTC) was introduced in 1960; its second corresponds to the atomic second, but the use of leap seconds keeps Coordinated Universal Time aligned with UT1. The International Earth Rotation and Reference Systems Service (IERS) publishes the daily difference between UT1 and UTC and announces the introduction of leap seconds. The following calculator uses IERS data to plot the difference between UT1 and UTC for a specified time period.
Difference between Universal Time and Coordinated Universal Time
Leap second data is obtained from the Paris Observatory website[^po]. A leap second may be added or subtracted on the last day of December or June. When a second is added, the UTC time reads 23:59:60; when one is subtracted, the final second is 23:59:58, followed immediately by the first second of the next day. The last leap second was added in December 2016. UTC is coordinated to ensure that the absolute difference between UT1 and UTC does not exceed 0.9 seconds. Currently, the UTC scale is used universally, including in computer systems. Introducing a leap second can cause errors in applications sensitive to uniform timekeeping, leading to active debate about discontinuing the practice; however, no decision has been reached so far.
Terrestrial Time (TT)
On January 1, 1958, International Atomic Time differed from Ephemeris Time by 32.18 seconds. With the advent of atomic clocks, Ephemeris Time began to be derived from atomic time rather than direct observation, as their time scales match. Ephemeris Time remained in use for astronomical calculations until 1984, when it was replaced by two slightly different scales: Terrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB). Terrestrial Dynamical Time retained the scale of Ephemeris Time; consequently, its second corresponds to one SI second. Due to the difficulty of defining TDB and TDT, they were replaced and, by 2006, completely superseded by the scales Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB). TCG and TCB reflect time rates adjusted in accordance with the general theory of relativity to clocks at rest relative to a reference point outside the gravitational influence of the Earth (for TCG) and the Solar System (for TCB). TCG and TCB are currently used for calculating the motion of celestial bodies. The scales of these time systems began to diverge from atomic time...
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