Date and time offset value

This is my first post to this group, so I hope that I did it correctly.

Here is my dilemma.  I change the system date and time, then I want to
have it automatically change back (before people have a chance to run
amok all over the wrong date/time).  So I come up with a time offset:
the difference in value between the original date/time that I started
with and the new date/time that I changed it to.  When I am ready to
change the system back, apply the negative of that offset to the time to
adjust it back to the correct time (plus the time passed).

Most of the calculations for converting date and time to minutes and
seconds that I came up with are pretty hairy, and I am looking for the
simplicity of a DATE-OF-INTEGER/INTEGER-OF-DATE solution.  Is there a
COBOL (or even a CL) function or API that will do the math for me
automatically?  None of the intrinsic functions in the COBOL/400
reference go deeper than date and I really need to include time: down to
the minute for necessity or down to the second for anal precision.  I
need two functions. . .one that takes two date/times and gives me the
offset between them to store, then something that allows me to take a
date/time and negates the offset to give me the time-passed original
date/time.

Thanks!!

--
Tom
Computer Service Systems