IOUs are usually documentary evidence written by civil subjects when they owe money or things to others. The IOU reflects the confirmation of creditor's rights and debts by both parties, but it can't explain the reasons of creditor's rights and debts.
The validity of IOUs refers to the statute of limitations of IOUs, which does not affect the validity of IOUs. As long as it is a legally signed, authentic and valid IOU, the IOU itself is valid no matter how long it takes.