"Correct" Ada compilers are OK until 2099--but only if no non-Ada code is
allowed to exchange dates with the Ada.
A similar caveat would apply to any other language/compiler that attempts
to solve the problem unilaterally.
I predict that by 2038, C and C++ will be dead, Ada significantly
different than it is now, Java completely unrecognizable, and inordinately
popular will be some yet-unimagined language whose name will be BASIC. :-)