Oracle 9i Enterprise Edition Release on W2K sp4.

I am trying to set up jobs with OEM in order to schedule export and backup.

It seems that with backup task there are no problems but with the export task I am in trouble with the schedule.
The form is not properly working and I can't choose or select a desired hours when the job has to start.
This is happening only with "day of the week" or "day of the month".
Every others possibility works (immediate and once).
Is this maybe a bug?

Thank you and kind regards.