When was Gregorian calendar right?

Well, when was it?

The actual length of sideric day, and of tropical day, increases
slowly, as tidal friction irreversibly slows down the rotation of
Earth.

The actual length of sideric year can change - the energy of Earth=B4s
orbit may change as energy is exchanged between orbital movement of
Earth and other planets. But those changes are said to be minor. The
changes in tropical year are likewise minor.

The result is that the number of tropical days in a tropical year
decreases.

Sometime in palaeozoic, tidal rhytmites allegedly show that there were
400 days in a year.

But the rate of tidal slowing is not constant. It changes with changes
in the configuration of shelf seas and ocean basins. There would be
major changes during ice ages, for example.

The true duration of tropical year is around 365,2423 to 365,2424
tropical days. Gregorian calendar requires 365,2425 days.

The cumulative error of Gregorian calendar through recent is thus less
than two days.

But how valid was Gregorian calendar in ice age?