Cron
To run a cronjob every 15 minutes:
5,20,35,50 * * * * /path/to/command
or
*/15 * * * * /path/to/command
To run a cronjob every 15 minutes:
5,20,35,50 * * * * /path/to/command
or
*/15 * * * * /path/to/command