如何用javascript判断录入的日期是否合法

function IsValidYear(psYear)
{
var sYear = new String(psYear);

    if(psYear==null)
    {
        return false;
    }

    if(isNaN(psYear)==true)
    {
        return false;
    }

    if(sYear == "")
    {
        return true;
    }

    if(sYear.match(/[^0-9]/g)!=null)
    {
        return false;
    }

var nYear = parseInt(sYear, 10);

    if((nYear < 0) || (9999 < nYear))
    {
        return false;
    }

return true;
}

function IsValidMonth(psMonth)
{
var sMonth = new String(psMonth);

    if(psMonth==null)
    {
        return false;
    }

    if(isNaN(psMonth)==true)
    {
        return false;
    }

    if(sMonth == "")
    {
        return true;
    }

    if(sMonth.match(/[^0-9]/g)!=null)
    {
        return false;
    }

var nMonth = parseInt(sMonth,10);

    if((nMonth < 0) || (12 < nMonth))
    {
        return false;
    }

return true;
}

function IsValidDay(psDay)
{
var sDay = new String(psDay);

    if(psDay==null)
    {
        return false;
    }

    if(isNaN(psDay)==true)
    {
        return false;
    }

    if(sDay == "")
    {
        return true;
    }

    if(sDay.match(/[^0-9]/g)!=null)
    {
        return false;
    }

var nDay = parseInt(psDay, 10);

    if((nDay < 0) || (31 < nDay))
    {
        return false;
    }

return true;
}

function IsValidDate(psYear, psMonth, psDay)
{
    if(psYear==null || psMonth==null || psDay==null)
    {
        return false;
    }

    var sYear = new String(psYear);
    var sMonth = new String(psMonth);
    var sDay   = new String(psDay);

    if(IsValidYear(sYear)==false)
    {
        return false;
    }

    if(IsValidMonth(sMonth)==false)
    {
        return false;
    }

    if(IsValidDay(sDay)==false)
    {
        return false;
    }

    var nYear = parseInt(sYear, 10);
    var nMonth = parseInt(sMonth, 10);
    var nDay   = parseInt(sDay,   10);

    if(sYear=="" && sMonth=="" && sDay=="")
    {
        return true;
    }

    if(sYear=="" || sMonth=="" || sDay=="")
    {
        return false;
    }

    if(nMonth < 1 || 12 < nMonth)
    {
        return false;
    }
    if(nDay < 1 || 31 < nDay)
    {
        return false;
    }

    if(nMonth == 2)
    {
        if((nYear % 400 == 0) || (nYear % 4 == 0) && (nYear % 100 != 0))
        {
            if((nDay < 1) || (nDay > 29))
            {
                return false;
            }
        }
        else
        {
            if((nDay < 1) || (nDay > 28))
            {
                return false;
            }
        }
    }
    else if((nMonth == 1) ||
            (nMonth == 3) ||
            (nMonth == 5) ||
            (nMonth == 7) ||
            (nMonth == 8) ||
            (nMonth == 10) ||
            (nMonth == 12))
    {
        if((nDay < 1) || (31 < nDay))
        {
            return false;
        }
    }
    else
    {
        if((nDay < 1) || (30 < nDay))
        {
            return false;
        }
    }

return true;
}