Question

For one or another reason this code sample gives 48 hours instead of 24, can anyone explain me why the UTC time of a new month in javascript bugs orw hat I am doing wrong? Thanks for helping me out.

 <script type="text/javascript">
    function myFunction()
    {
    var d = (Date.UTC(2012,07,01) - Date.UTC(2012,06,30)) / 1000 / 3600;
    alert(d);
    }
    myFunction();
 </script>

Answer 1

Because month 6 is July, since months are zero-based in JavaScript dates, and there are 31 days in July.

If you want the number of hours between July 1st and June 30th, you want:

var d = (Date.UTC(2012,06,01) - Date.UTC(2012,05,30)) / 1000 / 3600;

or more generally:

function hoursBetween(startYear, startMonth, startDay, endYear, endMonth, endDay){
  var d1 = Date.UTC(startYear, startMonth-1, startDay );
  var d2 = Date.UTC(endYear,   endMonth-1,   endDay   );
  return (d2-d1)/(3600*1000);
}

For more information, read the documentation on Date.UTC.