eraTdbtcb

[rv, tcb1, tcb2] = ERFA.tdbtcb(tdb1, tdb2)

Time scale transformation: Barycentric Dynamical Time, TDB, to Barycentric Coordinate Time, TCB.

Given:

   tdb1,tdb2  double    TDB as a 2-part Julian Date

   tcb1,tcb2  double    TCB as a 2-part Julian Date

              int       status:  0 = OK

tdb1+tdb2 is Julian Date, apportioned in any convenient way between the two arguments, for example where tdb1 is the Julian Day Number and tdb2 is the fraction of a day. The returned tcb1,tcb2 follow suit.
The 2006 IAU General Assembly introduced a conventional linear transformation between TDB and TCB. This transformation compensates for the drift between TCB and terrestrial time TT, and keeps TDB approximately centered on TT. Because the relationship between TT and TCB depends on the adopted solar system ephemeris, the degree of alignment between TDB and TT over long intervals will vary according to which ephemeris is used. Former definitions of TDB attempted to avoid this problem by stipulating that TDB and TT should differ only by periodic effects. This is a good description of the nature of the relationship but eluded precise mathematical formulation. The conventional linear relationship adopted in 2006 sidestepped these difficulties whilst delivering a TDB that in practice was consistent with values before that date.
TDB is essentially the same as Teph, the time argument for the JPL solar system ephemerides.

IAU 2006 Resolution B3

This revision: 2021 May 11