Auto Updating Date field script - Needs to udate date first time opened not each time

I have a for that has an auto updating date field using the following script: getField("Date").value = util.printd("dd-mmm-yyyy", new Date()); The problem is once the form is completed and saved under a different name it continues to update the date field every time it is opened. I need this form to udate the date only the first time it is opened any subsequent time the form is opened I need it to retain the original date from when the form was completed and saved.

Would anyone be able to provide some guidance as to how to alter this field or create a new field that will only set the date once.

I am using Adobe Acrobe Pro 2017 release date and the form needs to support digital signature

Any help would be greatly appreciated.