Hi Claude,
If you are creating reports using tables then go in Database menu and click on show SQL query, this will show the SQL generated by crystal reports. Now go in database expert and expend your connection and double click on Add Command and past the query which was generated by Crystal and try to modify the date format and click OK.
Now you need to redesign your reports based on Add command.
or
If your reports datasource is Add command you can directly go in add command and change it.
-Sastry