as far as associating dates, try formatting them to month, then linking them. IE: Month(GLBE_datum) as Month
Month(GLE_PostingDate) as Month.
To avoid synthetic keys, you can concat the data to create the link, and put this in both tables. Then use only the field from one table and rename them in the other. Then you can link the Bedrijf to GLBE by naming the fields the same, and link CostCenter to GLBE naming the fields the same.
This of course would match your field names to work.
BedrijfCode' & '-' & 'CostcenterCode' & '-' & Month(GLBE_datum) as mainlink