Google Excel Time Difference  
  function TimeDiff(cell1, cell2) {
        return ConvertMilliSecondsToTimeSpan(Math.abs(cell1 - cell2));
    }
    function ConvertMilliSecondsToTimeSpan(t) {
        var cd = 24 * 60 * 60 * 1000,
            ch = 60 * 60 * 1000,
            d = Math.floor(t / cd),
            h = '0' + Math.floor((t - d * cd) / ch),
            m = '0' + Math.round((t - d * cd - h * ch) / 60000);
        return [h.substr(-2), m.substr(-2), '00'].join(':');
    }
How to use it
| 1/1/2014 21:00:00 | 2/1/2014 1:15:00 | 
function usage 
=TimeDiff(E24,F24)