You could set your date to "auto populate" with today's date and then set the field value to "Read Only". This way the date would be whatever today's date is when the form opens, but the user would not be able to change it. You would auto populate the date by bringing onto your form the date object, going to "Tools", "Action Builder" and creating a new action with the Condition "When form (and select Form1) has finished loading, and choose the Result of "Set the value of "DateTimeField1" to today's date." OK, then set the field value to "Read Only" and it can't be changed.
We use this because some of our forms require an official "Date of Notice" to be whatever date the form is printed out.
Hope this helps.