This means that the initial holder of postdated checks cannot use them until the date on the checks has come.
This is not necessarily true, however, as there are some banks that will actually accept postdated checks and will deposit funds into the check holder's account in exchange for the postdated checks, even before the date of the check's date has come.
As mentioned earlier, banks will still accept postdated checks regardless of whether or not the date has come.
This is because postdated checks are essentially just negotiable instruments with a clause attached, where that clause does not affect the negotiability of the instrument.
In some instances, postdated checks will be treated as payable, regardless of their postdated status.
In such instances, one's checking account might be overdrawn, as the check might be paid before the drawer was ready to have it paid.
In this way, the drawer of a postdated check will be able to avoid any overdraft fees from the postdated check being drawn on a checking account without enough money in it to support payment of the check.
Postdated checks are perfectly legal, as long as they are not being used for illegal purposes.