SQL & PL/SQL

I want to schedule a job every half hour on the half or the hour. Using interval=>'sysdate + 1/48' does not work because each successive job is scheduled in a half hour plus a few seconds. Over time the schedule increasingly deviates from the half.

I scheduled a job successfully every hour on the hour with 'trunc(sysdate, ''HH24'') + 1/24'. Now how do I schedule a job every half hour on the hour or the half?

Thanks for your help.

Kevin Tyson, OCP