Adding relationship where start date exists but no day number is specified gives database error

Steps to reproduce error:

1. Go to OpenNGO sandbox, php4 version
2. Search for a contact, select one.
3. Click on relationships
4. Click "add a relationship"
5. Choose "employee of"
6. Search for "conservation corp" and select it
7. In the start date group, select a month and year, but leave the day of the month unselected.
8. Click "save relationship"

A non-recoverable database error occurs.
Workaround: supply only complete dates.