Calculate hour, minute in JavaScript or JQuery

Compare start time and end time with jquery. After comparing/calculating we will get the difference between start & end time.

HTML

<input id="a1" type="text" />
                     <input id="a2" type="text" onblur="calculate()"  />
                     <input id="a3" type="text" name="total_amt" />

JavaScript

calculate = function()
{	
  var start = document.getElementById('a1').value;
       var end = document.getElementById('a2').value;

	s = start.split(':');
	e = end.split(':');

	min = e[1]-s[1];
	hour_carry = 0;
	if(min < 0){
		min += 60;
		hour_carry += 1;
	}
	hour = e[0]-s[0]-hour_carry; 
	diff = hour + ":" + min;
  document.getElementById('a3').value = diff;
	//alert(diff);
	}