Perhaps a more canonical approach would be using date and stat:

Code:
cutoff=$((`date +%s`-64800))
age=`stat -c %Y "$testfile"`
# or, if your date command supports it:
# age=`date -r "$testfile" +%s`
if [ $age -gt $cutoff ]
then
    echo "File is too new."
    exit 1
fi
echo "File is over 18 hours old."