Use JavaScript to check the YYYY-MM-DD format for the correct source code.

A few days ago to see brother Liao about using JavaScript check YYYY-MM-DD format of the article, debugging a bit, found wrong, now give the correct source code, welcome to advise.
Program under the PWIN98,PWS, but there are two points do not understand, please enlighten us!
1, if using var Date1 = new Date (2000,12,31); , Date1 will get 2001,1,31.
The month automatically adds 1.
2, the format of new date (2000,12,31) is different from the new date (12,31,2000) format in the book, is it related to the date of the server?
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<script language=javascript>
<!--
function Form4_onsubmit ()
{
Ymd1=form4.date1.value.split ("-");
Month1=ymd1[1]-1
var Date1 = new Date (ymd1[0],month1,ymd1[2]);
if (Date1.getmonth () +1!=ymd1[1]| | Date1.getdate ()!=ymd1[2]| | Date1.getfullyear ()!=ymd1[0]| | YMD1[0].LENGTH!=4)
{
Alert ("Illegal date, please enter" in "YYYY-MM-DD" format);
Form4.date1.focus ();
return false;
}
Alert ("Enter Date:" + Form4.date1.value);
return true;
}
-->
</SCRIPT>
<% nowdate=year (date) & "-" &month (date) & "-" &day (date)%>
<form name=form4 method= "POST" action= "chkfind.asp" onsubmit= "return Form4_onsubmit ()" >
<input type= "text" name= "date1" size= "" Value=<%=nowdate%> >
</form>