I miss an indvidual date for the collections - not only the creation date of the collection!

Why is there only a "creation date" in the collections? I miss an individual "document date"! When I will start with 42Tags it is normally (for me) to scan "old" documents/contracts! But with the creation date it is not very useful to search for those collections! I have to remember the creation date of the collection and not the real document date!