I have created a protected form as a template, using dropdown fields, etc. I
want the date on which the form is used to fill automatically and remain as
this date, if reopened at a later date.

By using the current date field, the current day's date always appears. By
using the create date field, the date on which the form was created appears.
As the form is protected, I can't update the field.

I think there may be some way to protect, unprotect and insert the date and
reprotect the form? Any suggestions?