很着急，JS怎么计算周岁

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<!-- saved from url=(0011)about:blank -->
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; charset=unicode">
<META content="MSHTML 6.00.2900.3199" name=GENERATOR></HEAD>
<BODY><SCRIPT LANGUAGE="JavaScript">
<!--
function checkbirth(birth)
{
var date1=new Date(Date.parse(birth))
var date2=new Date()
var sum=date2.getFullYear()-date1.getFullYear()
date1.setYear(2000)
date2.setYear(2000)
if(date2<date1)
{
alert(sum+'周岁')
}
else
{
sum--
alert(sum+'周岁')
}
}
//-->
</SCRIPT>
<input type="text" value="4/17/1984" onblur="checkbirth(this.value)"></BODY></HTML>