Where is the federal source for the six month time period allowed to cash a check?
Actually, it's state law that you're looking for, in the form of the Uniform Commercial Code, a model statute that's adopted (and adapted) by each of the fifty states, the District of Columbia, and other U.S. jurisdictions. The law used to be that a check that was over six months old was called "stale-dated", and banks weren't supposed to pay them, so it was wise to get checks deposited or cashed sooner, rather than later, to avoid missing the deadline.
Today, the law reads differently. Recognizing that automated check processing usually doesn't provide an opportunity for banks to make a physical review of each check for the date, the law now says that a bank may (not must) refuse payment if a check is over six months old, but may also pay the check if it does so in good faith. That essentially means that if the bank doesn't notice that the check is more than six months old, it's OK to pay it, but it's still a good idea to get checks cashed or deposited early, to avoid the potential that they won't be paid if they are more than six months old.
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