Use creation date to generate the reference. This makes sure it is generated even if stop (or start) date is not available on the document. This also solve the potential issue of discrepancy between the source data (start or stop date) and the outcome (reference), since creation date is never editable.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@14738 20353a03-c40f-0410-a6d1-a30d3c3de9de