Answer by wanglabs for Detecting an "invalid date" Date instance in JavaScript

This function validates a string date in digit formats delimited by a character, e.g. dd/mm/yyyy, mm/dd/yyyy

/*
Param  : 
1)the date in string data type 
2)[optional - string - default is "/"] the date delimiter, most likely "/" or "-"
3)[optional - int - default is 0] the position of the day component when the date string is broken up via the String.split function (into arrays)
4)[optional - int - default is 1] the position of the month component when the date string is broken up via the String.split function (into arrays)
5)[optional - int - default is 2] the position of the year component when the date string is broken up via the String.split function (into arrays)

Return : a javascript date is returned if the params are OK else null
*/
function IsValidDate(strDate, strDelimiter, iDayPosInArray, iMonthPosInArray, iYearPosInArray) {
    var strDateArr; //a string array to hold constituents day, month, and year components
    var dtDate; //our internal converted date
    var iDay, iMonth, iYear;


    //sanity check 
    //no integer checks are performed on day, month, and year tokens as parsing them below will result in NaN if they're invalid
    if (null == strDate || typeof strDate != "string")
        return null;

    //defaults
    strDelimiter = strDelimiter || "/";
    iDayPosInArray = undefined == iDayPosInArray ? 0 : iDayPosInArray;
    iMonthPosInArray = undefined == iMonthPosInArray ? 1 : iMonthPosInArray;
    iYearPosInArray = undefined == iYearPosInArray ? 2 : iYearPosInArray;

    strDateArr = strDate.split(strDelimiter);

    iDay = parseInt(strDateArr[iDayPosInArray],10);
    iMonth = parseInt(strDateArr[iMonthPosInArray],10) - 1; // Note: months are 0-based
    iYear = parseInt(strDateArr[iYearPosInArray],10);

    dtDate = new Date(
        iYear,
        iMonth, // Note: months are 0-based
        iDay);

    return (!isNaN(dtDate) && dtDate.getFullYear() == iYear && dtDate.getMonth() == iMonth && dtDate.getDate() == iDay) ? dtDate : null; // Note: months are 0-based
}

Example call:

var strDate="18-01-1971";

if (null == IsValidDate(strDate)) {

  alert("invalid date");
}