At a basic level, writing a postdated check simply involves specifying a particular date in the date payable line in the upper-right corner. However, you'll also have to clearly communicate to the bank about the restriction on the check, or inform the recipient of the need to wait until that date to deposit or cash the check. Otherwise, there's usually nothing to stop him from taking it to the bank early and having it honored.
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Once you sign the check, it can legally be presented to the issuing bank in exchange for payment. This may be avoided by alerting your bank beforehand not to cash the check before the requested date. If you give oral notice, it's only valid for 14 days. After that, the bank can legally cash it even if the date on the check hasn't yet arrived. Making the request in writing can increase the period to six months. If either case, the bank could be liable for damages if it cashes a postdated check after you've told it not to. This notice, rather than the date itself, is what influences the bank's decision not to cash the check.
Writing a postdated check may put you in legal jeopardy if the check doesn't clear. All 50 states make it illegal to write a worthless check with the intent to defraud. That criminal intent is the key to making fraud charges stick. It's hard to prove intent, but if you postdate a check and then move all your money out of the account before the deposit date, the recipient would have a case against you.