Oracle Database Discussions

Hi,
could someone please help me :

I'd like to change a schedule interval on a job to run every 15 minutes on the hour but can't seem to get the syntax right.

this is what I have:

exec dbms_job.change (22,NULL,NULL,'SYSDATE +15/1440'); <----this works but it's not at the top of the hour.

sql>exec dbms_job.change (22,NULL,NULL,trunc(sysdate+1/24'HH'),'trunc(SYSDATE+15/24/60,''MI'')');
BEGIN dbms_job.change (22,NULL,NULL,trunc(sysdate+1/24,'HH'),'trunc(SYSDATE+15/24/60,''MI'')'); END;

*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character to number conversion error
ORA-06512: at line 1

thank you.