[2] Unlike former astronomical time scales, TCG is defined in the context of the general theory of relativity.
(The traditional values were in a sense wrong, incorporating corrections for the difference in time scales.)
For continuity with its predecessor Ephemeris Time, TCG was set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z).
Barycentric Coordinate Time (TCB) is the analog of TCG, used for calculations relating to the Solar System beyond Earth orbit.
When the Earth is at perihelion in January, TCG ticks even more slowly than it does on average, due to gravitational time dilation from being deeper in the Sun's gravity well and also velocity time dilation from moving faster relative to the Sun.