Crontab | random execution time

Dear list,
I am trying to run a cron job, starting once every two hours and at 
random minute  in the half our of that cron scheme:

SHELL=/bin/sh
PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:/usr/local/etc
0~30   */2     *       *       *       root /root/cronjobs/run_myjob

Unfortunately it is running exactly on the hour instead of random 
between 0-30 minutes.
Can you pls advice how to make this work?

Thanks
Jos Chrispijn