Assume you have a Registration form to register for a specific course and the registration is open till 30th Dec 2009. To restrict entries from being submitted after this date, use the zoho.currentdate variable within the on add -> on validate block of the Script tab, to validate if the registration date has expired. The sample code is given below.
if (zoho.currentdate > '20-Mar-2008')
{
alert "time for registration expired";
cancel submit;
}