Table.Filter: Filtering between dates - Help needed!!!

Hello newsgroup!

What in heaven's name is wrong with that filter statement?
Probably it's very simple but I can't find the mistake I made:

tbPersonal.Filter := 'EINTRITT BETWEEN '''+dbvon.Text+''' AND  
'''+dbbis.Text+''' OR EHEMALSSEIT BETWEEN '''+dbvon.Text+'''
AND  
'''+dbbis.Text+'';
tbPersonal.Filtered := True;

The error message I get is something about an incomplete term.

I would be thankful for every idea that helps.
Peter