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How can i calculate the difference between two dates?

The following JavaScript method provides this functionality: 

<html>
  <head>
    <script language="JavaScript">
      function y2k(number) {
        return (number < 1000) ? number + 1900 : number;
      }

      function daysElapsed(date1,date2) {
        var difference =
            Date.UTC(y2k(date1.getYear()),date1.getMonth(),date1.getDate(),0,0,0)
          - Date.UTC(y2k(date2.getYear()),date2.getMonth(),date2.getDate(),0,0,0);

        return difference/1000/60/60/24;
      }
    </script>
  </head>
  <body>
    <input type="button" value="Calculate difference" 
      onclick="alert(daysElapsed(new Date(2007, 02, 01), new Date(2007, 01, 01)) + ' days elapsed.');"/>
  </body>
</html>

Notice that you have to pass a Date object into the daysElapsed method and not String values.