Unfortunately cron isn't very flexible in that respect. Does your script need to run
exactly every 15 days, or could you just run it on the 1st and 16th of every month, i.e. approximately every 15 days?
Otherwise you might have to just run the job every day, and add some logic so that it exits without doing anything if the current number of days since Jan 1, 1970 is not evenly divisible by 15:
Code:
days_since_epoch=$(perl -e 'print int(time/86400)"\n";')
if [[ "$(( $days_since_epoch % 15 ))" -ne 0 ]]
then
exit
else
# do your stuff
fi
