Page "Leap year" Paragraph 5

The marginal difference of 0. 000125 days between the Gregorian calendar average year and the actual year means that, in 8, 000 years, the calendar will be about one day behind where it is now.

But in 8, 000 years, the length of the vernal equinox year will have changed by an amount that cannot be accurately predicted.

Therefore, the current Gregorian calendar suffices for practical purposes, and the correction suggested by John Herschel of making 4000 a non-leap year will probably not be necessary.