原子时
Time coordinates on the TCB scale are specified conventionally using traditional means of specifying days, inherited from slightly non-uniform time standards based on the rotation of the Earth. Specifically, both Julian Dates and the Gregorian calendar are used. For continuity with its predecessor Ephemeris Time, TCB was set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z). More precisely, it was defined that TCB instant 1977-01-01T00:00:32.184 corresponds exactly to the International Atomic Time (TAI) instant 1977-01-01T00:00:00.000, at the geocenter. This is also the instant at which TAI introduced corrections for gravitational time dilation.