People will typically post-date a check when they know there are not sufficient funds currently in the account to cover the check. They are waiting for a deposit to come in so that the check will not bounce. They may also post-date the check because that is when the bill is due. You need to be careful because when writing a check because it's legal tender. Just because there is a future date on the check, doesn't mean that a bank won't cash it on an earlier date.