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Function checks for valid date of the form dd/mm/yy or dd/mm/yyyy ( or ddmmyy, or d/m/yy etc.) Utilizes the fact that Javascript will internally convert dodgy dates. eg. 2002,4,44 is June 13, 2002 (remembering that Javascript months go from 0 to 11).

<!---Head--->
<script>
function stringToDate(inStr) { // returns a date from input dd/mm/yyyy
        if (!inStr) return false;
        var reDate = /^s*(d{1,2})D*(d{1,2})D*(d{2,4})s*$/
        var matchArray = reDate.exec(inStr);
        if (!matchArray) return false;
// to check for mm/dd/yyyy, swap these next two lines
        var tDay   = parseInt(matchArray[1],10);
        var tMonth = parseInt(matchArray[2],10);
        var tYear  = parseInt(matchArray[3],10);
        if (isNaN(tDay + tMonth + tYear)) return false;
        if (tYear.toString().length == 3) return false;
        if (tYear.toString().length <= 2) 
            tYear += (tYear < 50) ? 2000 : 1900;

        return isValidDate(tYear, tMonth - 1, tDay);
}

function isValidDate(inY,inM,inD) {
       var testDate = new Date(inY, inM, inD);
       if (!testDate) return false;
       return (    testDate.getDate() == inD 
                && testDate.getMonth() == inM 
                && testDate.getFullYear() == inY) ? testDate : false;
}
</script>

<!---Body--->

<form>
<input type=text onchange="dateObject=stringToDate(this,value);">
</form>