subtracting one time from another

Question

I have had a good search for what I'm looking for but can't seem to find exactly what I need. I'm new to jQuery and JavaScript.

All I want to do is subtract one 24 hour clock time from another to find how many hours have been allocated using JQuery or JavaScript i.e. start time: 12:00 end time: 16:00 would be 4 hours.

How would I go about doing this without having issues when going from say 12:00 till 12:00 the following day?

I am not dealing with with dates, just time.

Currently the times are stored like this as part of an object with start_time end_time:

var shift = {'location':$('#shift_location').val(),
             'shift_date': $('#shift_date').val(),
             'start_time':$('#shift_start_time').val(),
             'end_time':$('#shift_end_time').val()
            };
var shift_list = JSON.parse(localStorage.shift);
shift_list.push(shift);
localStorage.shift = JSON.stringify(shift_list);

http://stackoverflow.com/questions/1944810/javascript-subtracting-time-and-getting-its-number-of-minutes might be useful — wjhguitarman, Jan 17 '14 at 19:44
Checkout using Momentjs http://momentjs.com/. It has the features you need. — xspydr, Jan 17 '14 at 19:44
How are the times stored? Is it a string like `'12:00'`, a Date object, something else? — Andrew Clark, Jan 17 '14 at 19:47
if times span multiple days you'll have to use the date instead of just the times. — John Boker, Jan 17 '14 at 19:50
yes it is stored as a string, thanks all for the links and info, was very helpful. — JamesF, Jan 17 '14 at 20:28

score 1 · Answer 1 · answered Jan 17 '14 at 20:05

1

You can use momentjs to do things with dates in javascript.

Example (moment doc, fiddle):

var start_time = "12:00";
var end_time = "16:00";

var start = moment.duration(start_time, 'h');
var end = moment.duration(end_time, 'h');
alert(end.subtract(start).hours()); // 4

Of course, because of the simplicity of the task you could always use plain javascript:

var start_time = "12:00";
var end_time = "16:00";
alert(parseInt(end_time, 10) - parseInt(start_time, 10)); // 4

answered Jan 17 '14 at 20:05

R. Oosterholt

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If you're new to JavaScript you should use a good library like momentjs for date time calculations and date parsing. – skmasq Jan 17 '14 at 20:16
thanks this is almost perfect, however if I use a time with minutes it doesn't add them up, just the hours. I imagine this is to do with the .hours so I should be able to figure it out. This answer has put me on the right track, cheers – JamesF Jan 17 '14 at 20:34

score 1 · Accepted Answer · edited May 23 '17 at 12:03

With the given information i.e start_time and end_time there is no way you can cover multiple days. They just oscillate between 0 to 23 hours. There is no counter involved to calculate multiple days. if you need that you need two more states which will store start_date and end_date which will act as counter as pointed by @John Boker. But if you are sure that the difference never goes beyond 24 hours then we can use the parseTime from JAVASCRIPT: subtracting Time and getting its number of minutes function with our own little modifications.

function parseTime(s) {
   var c = s.split(':');
   return parseInt(c[0]) * 60 + parseInt(c[1]);
}

var limit = parseTime("23:59");

function getDiff(start_time, end_time){
   var a = parseTime(start_time), b = parseTime(end_time);
   if(b < a) // means its the next day.
      return Math.round((limit - a + b)/60);
   else if(b > a)
      return Math.round((b - a)/60);
   else if(b - a == 0)
      return 24.0;
   else
      alert("Invalid data");
 }

 alert(getDiff("12:00", "11:00"));

Thanks this is explained really well, I shall use this! – JamesF Jan 18 '14 at 19:37 — JamesF, Jan 18 '14 at 19:37

subtracting one time from another

2 Answers2