Crontab Formatting - every 15 minutes

Crontab doesn't remember what time you "started" (presumably the time you executed the crontab -e or crontab filename command).

If you want to run the job every 15 minutes starting from an arbitrary time, you'll have to specify that time. This:

7-59/15 * * * * command

will run at 7, 22, 37, and 52 minutes after each hour. That's assuming you're running Vixie cron, which is the most common implementation. For better portability, you can use:

7,22,37,52 * * * * command

And remember that you can't have spaces within any of the first 5 fields; 0, 15,30,45, as you had in your question, is invalid.

You would format the crontab like this to get it to run every 15 minutes.

*/15 * * * * [path/to/script]

0,15,30,45 for 15 minutes is incorrect.

The better and simply way is */15 * * * * for 15 minutes.

5 minutes
*/5 * * * *

15 minutes
*/15 * * * *

30 minutes
*/30 * * * *

60 minutes
0 * * * *

1 day
0 0 * * *