Please tell me how to create DBMS_JOB?

I have tried the following:
to execute the procedure 'procedure_name' every 1/2 hour as an admin user.


SQL> variable jobno NUMBER;
SQL>begin
>DBMS_JOB.SUBMIT(:jobno,'procedure_name',sysdate,'sysdate+24/48');
>end;

It gave error: ORA:00900.

what is the problem with the syntax?

I have tried

exec DBMS_JOB.SUBMIT(:jobno,'procedure_name',sysdate,'sysdate+1');

also.

Please suggest me exact syntax and any settings to be done.