If you use transaction logging and use SETCLOCK ;NOW while the logging
processes are running.  The log files will have time stamps the are
naturally increasing in time, then fallback appearing to start over and
then increase in time. It may cause a recovery to fail because of the
apparent "out-of-sequence" timestamps.   Likewise, if you have programs
that scan the system log files, the same can happen.    And then on your
application side,  what about timestamps that you might be using in the
application.

SETCLOCK ;GRADUAL slows the clock down, such that time continues, but at a
slower pace, timestamps never fall back.

In the spring the opposite is occurs, the ;NOW option doesn't cause as much
potential problems UNLESS you use a CPU billback program.  Then some poor
user, really becomes much poorer.  The program that happens too be running
when the clock changes gets nailed for 1 hour elapsed (and possibly CPU)
time.