Jquery time difference in hours from two fields

I have two fields in my form where users select an input time (start_time, end_time) I would like to, on the change of these fields, recalcuate the value for another field.

What I would like to do is get the amount of hours between 2 times. So for instance if I have a start_time of 5:30 and an end time of 7:50, I would like to put the result 2:33 into another field.

My inputted form times are in the format HH:MM:SS

So far I have tried...

$('#start_time,#end_time').on('change',function() 
{
  var start_time = $('#start_time').val();
  var end_time = $('#end_time').val();

    var diff =  new Date(end_time) - new Date( start_time);  



  $('#setup_hours').val(diff);