For example, a document dated 1/4/20 can easily be changed to 1/4/2021 by adding two numbers at the end.
There are several ways that could pose a problem. Rheingold cited the example of a stale check, or one that was written more than six months or so ago. If you have an old check lying around that's dated 1/4/20 and someone finds it, they could add "21" to the end of that date, and voila, the check is no longer stale.
Or, let's say you sign a credit contract — an agreement between a borrower and a lender — and date it 1/4/20. Say you then miss a month or two of payments, and the lender goes to collect the debt that's owed. Theoretically, they could add "19" to the end of that date and argue that you owe more than a year's worth of payments, Rheingold said.