You haven't defined what Date is. I think you want o do somethig like this:
var nDate = new Date();
SubmitDate.rawValue = nDate.getDate();
After this code runs the value of the current day (an integer between 1-31) will be placed in the SubmitDate field. For today it woudl return 6.
Note that this is standard javascript and has nothing to do with the form. The only part that is unique to the form is the assignment statement to SubmitDate.
Paul