"Scheduled Run Date" can't be set when creating new scheduled job

To reproduce:

• visit civicrm/admin/job?action=add&reset=1, enter a time, and submit the form
• inspect the database and find that the field is NULL

The cause:
Timestamp fields are explicitly excluded from $dao->save() operations.

The conservative fix:
Update CRM/Admin/Form/Job.php. Execute a second query (not using $dao->save()) to save the timestamp after the initial insert. (Actually, there is too much business logic in this form. That logic should be moved to a BAO and the fix should be made there, thinning out the form layer.)

The fix that seems more right but which will probably break a bunch of stuff:
How arbitrary that timestamps can't be saved via the DAO! Not all timestamp fields are "on update CURRENT_TIMESTAMP" fields. Strip that junk out!