You need to trust the payee to not cash it until the postdate, or that the bank they use will honor the date if the check is deposited.
You can try to cash a postdated check early, but it may or may not work.
Banks may be willing or simply ignore the date, and, as mentioned, aren’t legally obligated to delay unless they have explicit, often written instructions from the writer.
Many bigger banks, however, will honor postdates as part of banking agreements. When it comes down to it, postdating a check can be risky.
Do not be surprised if they do not allow it, however, even if it is in line with state laws.
If you are the payee, cashing a postdated is the same as cashing any other type of check. Instead, you may want to deposit the check, as banks will often allow you to make the transaction, but not actually deposit the money until the postdate.