Time and Frequency from A to Z:  Dr to E

The linear (first order) component of a systematic change in frequency of an oscillator over time. Drift is caused by  aging, by changes in the environment, and by other factors external to the oscillator.

The current difference between UTC and to the astronomical time scale UT1.  It is always a number ranging from -0.8 to +0.8 seconds, with a resolution of 0.1 seconds.  This number is broadcast by WWV, WWVH, WWVB, and ACTS, and can be added to UTC to obtain UT1.  The current DUT1 correction is available here.

A group of clocks or oscillators whose outputs are averaged to create a time scale.  Typically, the relative value of each clock is weighted, so that the best clocks contribute the most to the average.  NIST uses an ensemble of clocks to produce UTC(NIST).

An obsolete time scale based on the ephemeris second, which served as the SI second from 1956 to 1967. The ephemeris second was a fraction of the tropical year, or the interval between the annual vernal equinoxes, which occur on or about March 21. The tropical year was defined as 31,556,925.9747 ephemeris seconds. Determining the precise instant of the equinox is difficult, and this limited the uncertainty of Ephemeris Time (ET) to +/- 50 ms over a 9-year interval. ET was used mainly by astronomers, and was replaced by Terrestial Time (TT) in 1984.

The beginning of an era (or event) or the reference date for a system of measurements.