Center block with CSS

Hello expert,

I have read several places that to center a block
with css one should use
margin-left: auto; margin-right: auto;
but it doesn't seem to work in the code below.

Thanks

Allen in Dallas

PS
In IE 8 the Javascript the date writes but the centering does not work.
In FireFox 30 the centering works but the JavaScript date does not write.

++++++++begin code++++++++++++++++

<HTML>
<HEAD>
<title> JS Date </title>
<style type="text/css">
div {font-family:arial; color:sienna;}
</style>
</HEAD>
<BODY>
<font face="arial" size="4">This writes
the date. <br />It is an example of inline Javascript.</font></br >
<div style = "background-color: yellow; width: 600px; height: 200px; margin-left: auto; margin-right: auto;">

<div style = "background-color: #004280; color: #FFFFFF; width: 400px; height: 100px; text-align: center; margin-left: auto; margin-right: auto;">

<script type="text/javascript">
window.onload=function(){

var now = new Date();
document.getElementById('newDateID').innerHTML = now;
}
</script>
<p id = "NewDateID"></p>
</div>
</div>

</body>
</html>
++++++++end code++++++++++++++++
Allen PittsBusiness analystAsked:
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Trenton KnewOwner / Computer WhispererCommented:
in order for this to work, you need to define the width of the parent container, (in this case, the body tag)  

<body style="width:100%"> or <body style="width:!Valuepx">

if that still doesn't work, you might need a position:relative and/or to contain the entire page in a <div>
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Trenton KnewOwner / Computer WhispererCommented:
also, check your case on the div ID.  the java code calls for "newDateID" but the element name is "NewDateID"
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