Calculate in time format

Question

I need help creating a time calculation. I have a field:

Hora inicial <input id="inicial")>
Hora final <input id="final">
Total de horas <input id="resultado">

I can add integers(00), but how it looks in hours format(00:00) gives error the calculation.

I know very little Javascript, someone can give a force?

I tried that:

<script type="text/javascript"> function calcular(){
   var n1 = parseInt(document.getElementById('n1').value, 10);
   var n2 = parseInt(document.getElementById('n2').value, 10);
   document.getElementById('resultado').innerHTML = n1 - n2;   
} 
</script>

If I pass a value as follows: 14:00 to n1 16:00 to N2

It does not display the 2 that is the result.

I want to know how long between 2:00 and 4:00.

I need to know only the difference on the same day, will not pass 24 hours so I can not pass date and return time. as suggested.