Though you should do it as soon as possible to allow the bank time to process the instructions, and you might have to pay a fee. Federal law does not provide a minimum time period within which you must notify the bank. To instruct the bank not to issue the funds early, both state and federal banking laws normally require that you give the bank "reasonable" notice of the postdating. The bank can cash your post-dated check at any time, even before its authorized date -unless you specifically instruct it otherwise.