A time-scale used for calculating geocentric positions of Solar System bodies as an approximation to the relativistic time-scale Barycentric Dynamical Time (TDB). It is a continuation of Ephemeris Time. TT was introduced in 1984 under the name Terrestrial Dynamical Time (TDT) but was renamed in 1991. The fundamental unit of TT is the day of 86 400 SI seconds. TT is related to International Atomic Time (TAI) through the definition that 1977 January 1.0 TAI corresponds to 1977 January 1.0003725 TT. This means that TT runs permanently 32.184 seconds ahead of TAI, but behind UTC, which moves further ahead of TT each time a leap second is introduced.