Hi Dany,
You can not directly add dates in this manner.
Here is a rough script, that should help you.
var startDate = new Date(crmForm.all.new_startdate.DataValue);
startDate.setDate(startDate.getDate() + crmForm.all.new_delaydays.DataValue);
crmForm.all.new_enddate.DataValue = startDate;
I haven't tried this in a CRM environment, but this is how it should go, minor changes in syntax might be needed.
Regards,
Chinmay
http://metrix.blogspot.com