Javascript date validation

This code shows how to validate date using javascript

// Date Validation
function checkDate(sDate)
{
    var len = sDate.length;
    if(sDate != "")
    {
        if(len > 11)
        {
            alert("Not a Valid Date");
            return false;
        }
        else
        {
            var dateformat = /((([0][1-9]|[12][\d])|[3][01])[-\/]([0][13578]|[1][02]|(Jan|Mar|May|Jul|Aug|Oct|Dec))[-\/][1-9]\d\d\d)|((([0][1-9]|[12][\d])|[3][0])[-\/]([0][13456789]|[1][012]|(Jan|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec))[-\/][1-9]\d\d\d)|(([0][1-9]|[12][\d])[-\/]([0][2]|[2]|(Feb))[-\/][1-9]\d([02468][048]|[13579][26]))|(([0][1-9]|[12][0-8])[-\/]([0][2]|[2]|(Feb))[-\/][1-9]\d\d\d)/i
            if( ! dateformat.test(sDate))
            {
                alert("Not a Valid Date");
                return false;
            }
            else
            {
                if(len == 11)
                {
                    var month = sDate.substring(3, 6);
                    var month1 = /(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)/i
                    if( ! month1.test(sDate))
                    {
                        alert("Not a Valid Date");
                        return false;
                    }
                }
            }
        }
    }
    return true;
}