In Products file I've added 12 Date fields to a Layout.
But in what Table are they defined? If you deinfed these in Contacts, you will see the same dat every time you link a given products record to that customer, you'll see the same dates. That's one very likely explanation here. (A given Layout can have fields from both tables here.)
You've described this in terms of two files. Do you really have two files or just one file with two tables?