Openshift v1 does not allow custom cron times, cronjobs can only be placed in the folders minutely, hourly, daily, weekly and monthly.
You can use the script below to run a cronjob every 5 minutes. You can vary the timestamp by modifying the +4 parameter. The script must be placed in the minutely folder. Here it will run every minute, and write a timestamp to a temporary file. It will check the creation date of the file, and if it is more than 4 minutes ago the cronjob will run. After that the file will be removed and recreated via the touch command.
The example below will run a PHP cronjob.
#!/bin/bash
if [ ! -f $OPENSHIFT_DATA_DIR/last_run ];
then
touch $OPENSHIFT_DATA_DIR/last_run
cd $OPENSHIFT_REPO_DIR;php cronjob.php RememberCron
else
if [[ $(find $OPENSHIFT_DATA_DIR/last_run -mmin +4) ]]; then #run every 5 mins
cd $OPENSHIFT_REPO_DIR;php cronjob.php RememberCron
rm -f $OPENSHIFT_DATA_DIR/last_run
touch $OPENSHIFT_DATA_DIR/last_run
fi
fi