Change the last modified time of a file
This script will change the last modified time of a file in the current directory to 4 days back.
#!/bin/ksh
numDays=4
diff=86400*$numDays
export diff
newDate=$(perl -e 'use POSIX; print strftime "%Y%m%d%H%M", localtime time-$ENV{diff};')
lastFile=$(ls -lt | egrep -v ^d | tail -1 | awk ' { print $9 } ')
touch -t $newDate $lastFile