http://developer.irt.org/script/1.htm
<script language="JavaScript"><!--
function y2k(number) { return (number < 1000) ? number + 1900 : number; }
function isDate (day,month,year) {
// checks if date passed is valid
// will accept dates in following format:
// isDate(dd,mm,ccyy), or
// isDate(dd,mm) - which defaults to the current year, or
// isDate(dd) - which defaults to the current month and year.
// Note, if passed the month must be between 1 and 12, and the
// year in ccyy format.
var today = new Date();
year = ((!year) ? y2k(today.getYear()):year);
month = ((!month) ? today.getMonth():month-1);
if (!day) return false
var test = new Date(year,month,day);
if ( (y2k(test.getYear()) == year) &&
(month == test.getMonth()) &&
(day == test.getDate()) )
return true;
else
return false
}
if (isDate(31,2,1997))
document.write("Valid");
else
document.write("Invalid");
//--></script>
Update: dem snowman sein script ist aber besser
