This Drupal 7 module was created as the solution for this forum request.

The use case

Have three fields:

expiry - data field, format d-m-Y
reminder data field, format d-m-Y
ddays - select the numerical list - 30 days -60 days - 90 days 120 days

When the user enters or changes the expiry field the reminder data field must be updated to: expiry- dday