Your best bet might be to take advantage of this script's ability to display more than one countdown. If you follow the instructions on the demo page exactly, you will have two countdowns. Configure each as you want it to be as far as formatting and date go, but set one to be display none (example using the code from the demo page, addition highlighted):
Code:
<div id="countdowncontainer"></div>
<br />
<div id="countdowncontainer2" style="display: none;"></div>
<script type="text/javascript">
var futuredate=new cdtime("countdowncontainer", "March 23, 2009 18:25:00")
futuredate.displaycountdown("days", formatresults)
var currentyear=new Date().getFullYear()
//dynamically get this Christmas' year value. If Christmas already passed, then year=current year+1
var thischristmasyear=(new Date().getMonth()>=11 && new Date().getDate()>25)? currentyear+1 : currentyear
var christmas=new cdtime("countdowncontainer2", "December 25, "+thischristmasyear+" 0:0:00")
christmas.displaycountdown("days", formatresults2)
</script>
Then have your button simply toggle the display value of your two countdowns:
Code:
<input type="button"
onclick="var c1 = document.getElementById('countdowncontainer').style,
c2 = document.getElementById('countdowncontainer2').style;
c1.display = c1.display === 'none'? 'block' : 'none';
c2.display = c2.display === 'none'? 'block' : 'none';"
value="Toggle Dates">
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