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;
}