  The evolution of a solar model is initialized with homogeneous models of
either, pre-main sequence (P-models) or, zero-age main sequence (Z-models). The
zero-age of a solar model is conventionally referenced as the time where the
nuclear reactions just begin to dominate gravitation as the primary source of
energy. Fixing the physics, we found that the structure of P- and Z-models
computed with the same physics are almost similar soon after the exhaustion of
their convective core. This similarity gives a connection between the age of
the Sun t_sun and the time t_cal elapsed in the calculation of calibrated solar
models. We found that a Z-model calibrated with t_cal=t_sun and a P-model
calibrated with t_cal=t_sun+25My, are indistinguishable at the relative
accuracy level of a few 10^{-4}.
