historical difference between TAI and ET when TT was introduced, the TAI realization of TT is defined thus: TT(BIPM) Because TAI is never revised once published, it is possible for errors in it to become known and remain uncorrected. Approximately annually since 1992, the International Bureau of Weights and Measures (BIPM) has produced better realizations of TT based on reanalysis of historical TAI data. BIPM's realizations of TT are named in the form "TT(BIPM08)", with the digits indicating the year of publication. They are published in the form of a table of differences from TT(TAI), along with an extrapolation equation that may be used for dates later than the table. The latest is TT(BIPM21). Others Researchers from the International Pulsar Timing Array collaboration have created a realization of TT based on observations of an ensemble of pulsars. This new pulsar time scale is an independent means of computing TT, and it may eventually be useful to identify defects in TAI. Approximation Sometimes times described in TT are used in situations where TT's detailed theoretical properties are not significant. Where millisecond accuracy is enough (or more than enough), TT can be summarized in the following manners: To millisecond accuracy, TT is parallel to the atomic timescale (International Atomic Time, TAI) maintained by the BIPM. TT is ahead of TAI, and can be approximated as TT ≅ TAI + 32.184 seconds. (The offset 32.184 s arises from the history.) TT is also parallel with the GPS time scale, which has a constant difference from atomic time (TAI − GPS time = +19 seconds), so that TT ≅ GPS time + 51.184 seconds. TT is in effect a continuation of (but is more precisely uniform than) the former Ephemeris Time (ET). It was designed for continuity with ET, and it runs at the rate of the SI second, which was itself derived from a calibration using the second of ET (see, under Ephemeris time, Redefinition of the second and Implementations). TT is slightly ahead of UT1 (a refined measure of mean solar time at Greenwich) by an amount known as ΔT = TT − UT1. ΔT was measured at +67.6439 seconds (TT ahead of UT1) at 0h UTC on 1 January 2015; and by retrospective calculation, ΔT was close to zero about the year 1900. The difference ΔT, though somewhat unpredictable in fine detail, is expected to continue to increase, with UT1 becoming