I need directions on "Text Alignment"

I have used this code to greet user:

<script type="text/javascript">
var d = new Date();
var time = d.getHours();
if (time > 17) 
{
document.write("<b>Good Evening, Thanks For Visiting Us!</b>");
}
else if (time <17 && time >12)
{
document.write("<b>Good Afternoon, Thanks For Visiting Us!</b>");
}
else
{
document.write("<b>Good Morning, Thanks For Visiting Us!</b>");
}
</script>

Its working fine for me, but the problem is

This code writes "Greeting Message" in the left side of the page & i want the Message

To be displayed in the Center(middle) of the page.

What else should i add to the existing code to get it work.

Thanks all.