All personal computers will likely to be 64-bit by the year 2038.

But many embedded systems may still be 32-bit or less.

Note that the large majority of embedded systems today are still 8 or 16-bit. These could include microwave ovens, wrist-watches, elevators, gas-station pumps, car fuel injection computers, radios etc.

There are orders of magnitudes more small embedded systems in the world than there are desktop computers.

Also, application software running on 64-bit systems may not use the POSIX time_t type correctly. For instance, a C programmer may inadvertently cast time_t to 32-bits during a time calculation; or implicitly knock off the top 32 bits of a 64-bit time while storing or retrieving the time.

For authoritative information on the Unix 64-bit question, visit:

   http://www.unix.org/version2/whatsnew/login_64bit.html
   http://www.unix.org/version2/whatsnew/lp64_wp.html

For authoritative information on the year-2038-bug by the same site, visit:

   http://www.unix.org/questions_answers/faq.html#30
   http://www.unix.org/version2/whatsnew/year2000.html

(I.e. they don't say much.)