One of the jobs we completed in a hurry a few years ago was for an organisation that made an assumption that the open date and closed date for the contract files was the beginning and ending date of the contract.
Wrong! The open date is the date of the first piece of paper on the file (I'm only thinking physical files here). The closed date is the date of the last piece of paper in the file. Ideally the pieces of paper are filed chronologically but that is not always the case.
We set up a conveyor belt sort of system with ten Squad members working. The off site storage boxes were brought up, files were opened, details hand written on the list that had been provided by the client and each day a batch of paper was couriered back to their office.
The retention period is decided based on the closed date so it is critical to get that information right.