The current fundamental relativistic time scales are Geocentric Coordinate Time ( TCG ) and Barycentric Coordinate Time ( TCB ); both of these have rates that are based on the SI second in respective reference frames ( and hypothetically outside the relevant gravity well ), but on account of relativistic effects, their rates would appear slightly faster when observed at the Earth's surface, and therefore diverge from local earth-based time scales based on the SI second at the Earth's surface.
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:We have an article on this ( but it is badly in need of references ) Geocentric Coordinate Time for just such a time keeper : " TCG ticks faster than clocks on the surface of the Earth by a factor of about 7.0 ?10 " 10 ( about 22 milliseconds per year ) . " Which is 0.7 nanoseconds less than our second ( proper time ) or, in other words, our clocks are about 0.7 nanoseconds per second slower than TCG's proper time.
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It was eventually realized that TDB was not well defined because it was not accompanied by a general relativistic metric and because the exact relationship between TDB and TDT had not been specified . ( It was also later criticized as being not physically possible in exact accordance with its original definition : among other things the 1976 definition excluded a necessary small offset for the initial epoch of 1977 . ) After the difficulties were appreciated, in 1991 the IAU refined the official definitions of timescales by creating additional new time scales : Barycentric Coordinate Time ( TCB ) and Geocentric Coordinate Time ( TCG ).
