14694 years

[T. Ment]

The DESQview installation check overloads the set system date interrupt
(21h/2bh) with an invalid year (> 2099). Even if you patch DOS to handle
later years, other software relies on the same trick, with CX values as
low as 4149h, according to RBIL.

4149h is 16713 decimal, so 14694 years from now, DOS will need a rebased
calendar.

[JJ]

Post by T. Ment
The DESQview installation check overloads the set system date interrupt
(21h/2bh) with an invalid year (> 2099). Even if you patch DOS to handle
later years, other software relies on the same trick, with CX values as
low as 4149h, according to RBIL.
4149h is 16713 decimal, so 14694 years from now, DOS will need a rebased
calendar.

That won't even be applicable, because the CMOS only supports 4 digits year.
So, even if DOS is modified to support year > 2099, it can't get pass
through year 9999 reliably unless the system is never rebooted.