Sometimes it's hard to beat Perl. I would use Perl's File::Find for the whole thing but here's something that will create a touch timestamp spec for you - a string in the format CCYYMMDDhhmm.ss that can be -p seconds before or -n seconds after now.
Use it like this:
#!/usr/bin/sh
typeset TMSPEC=''
typeset TDIR=${TMPDIR:-/var/tmp}
typeset REFFILE=${TDIR}/X${$}_1.ref
TMSPEC=$(timestamp.pl -p 300) # 300 seconds before now
touch -m -t ${TMSPEC} ${REFFILE}
# your find stuff goes here
rm -f ${REFFILE} # clean up after yourself
Here's the Perl attachment, timestamp.pl;
If it ain't broke, I can fix that.