If you simply need to display the result of this date calculation in a report or form, you can create a SELECT query and insert an "immediate-if statement" (IIF). Here's how...
Let's say your "further date" field is called "Further_Date" and your original date field is called "My_Date". Create a query with your table(s) and in one of the columns in the query design grid, put this formula (changing the field names appropriately):
Translated, this says if some date was entered in the Further_Date field, subtract My_Date from Further_Date and display the result; otherwise, subtract My_Date from today's date. Everything before the first comma is criteria, what comes after it is what to do if the criteria is TRUE, and what comes after the second comma is what to do if the criteria is FALSE. Date() is Access' command for getting today's date.
This won't change any data in your table(s), but you can use the SELECT query in a report or to display the result on a form. There is a way to use it in a special query that changes data in tables, but you'll need some formal Access classroom instruction before you attempt that (you can easily destroy your data).
This formula should work for display and reporting purposes so long as Further_Date and My_Date are of data type "Date" in the table. If you get an error message, post it here.
In your query, this formula will be titled "Expr1:". Change that title to whatever you like, but you must leave the colon : at the end.