  The modern Very Long Baseline Interferometry (VLBI) relativistic delay model,
as documented in the IERS Conventions refers to the time epoch when the signal
passes one of two stations of an interferometer baseline (selected arbitrarily
from the pair of stations and called the 'reference station', or 'station 1').
This model consists of the previous correlation procedure used before the year
2002. However, since 2002 a new correlation procedure that produces the VLBI
group delays referring to the time epoch of signal passage at the geocenter has
been used. A corresponding correction to the conventional VLBI model delay has
to be introduced. However, this correction has not been thoroughly presented in
peer reviewed journals, and different approaches are used at the correlators to
calculate the final group delays officially published in the IVS database. This
may cause an inconsistency up to 6 ps for ground-based VLBI experiments between
the group delay obtained by the correlator and the geometrical model delay from
the IERS Conventions used in data analysis software. Moreover, a miscalculation
of the signal arrival moment to the 'reference station' could result a larger
modelling error (up to 50 ps). The paper presents the justification of the
correction due to transition between two epochs elaborated from the Lorentz
transformation, and the approach to model the uncertainty of the calculation of
the signal arrival moment. The both changes are particularly essential for
upcoming broadband technology geodetic VLBI observations.
