If the date is actually stored as a Date (and not Text), this is pretty easy.
Access actually stores Dates as numbers, specifically the number of days since 1/0/1900.
So all you need to do to find the difference between two dates is subtract them.
On your Form, if your Date field was named Date1, then you would add a Text Box to the Form, and use this formula in the Control Source of that Text Box:
Code:
=DateSerial(2017,12,31)-[Date1]
Note that this will return 244. If you want the date inclusive, just add 1, i.e.
Code:
=DateSerial(2017,12,31)-[Date1]+1