Get week number with week starting from sunday

Question

I currently have this code where I get the week number alongside the start/end day of that week with add and substract buttons:

Date.prototype.getWeekNumber = function() {
  var d = new Date(Date.UTC(this.getFullYear(), this.getMonth(), this.getDate()));
  var dayNum = d.getUTCDay() || 7;
  d.setUTCDate(d.getUTCDate() + 4 - dayNum);
  var yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
  return Math.ceil((((d - yearStart) / 86400000) + 1) / 7);
};

var curr;
weekReset();

function display() {
  start.textContent = curr;
  end.textContent = endOfWeek(curr);
  week.textContent = ("Week " + curr.getWeekNumber());
}

function weekReset() {
  curr = startOfWeek(new Date());
  display();
}

function startOfWeek(date) {
  var start = new Date(date);
  start.setHours(0, 0, 0, 0);
  start.setDate(start.getDate() - start.getDay());
  return start;
}

function endOfWeek(date) {
  date = startOfWeek(date);
  date.setDate(date.getDate() + 6);
  return date;
}

function weekPlus(weeks) {
  curr.setDate(curr.getDate() + 7 * weeks);
  display();
}

<div id="start">start</div>
<div id="end">end</div>
<div id="week">week</div>
<button onclick="weekReset()">current</button>
<button onclick="weekPlus(1)">add</button>
<button onclick="weekPlus(-1)">substract</button>

The problem is that it show's the current week number as 34;

starting on Sun Aug 27

ending on Sat Sep 02

... when this should be the week number 35.

So how can I change this function to count sunday as the first day of the week and saturday as the last day of the week?

Any help is appreciated.

score 3 · Accepted Answer · answered Aug 28 '17 at 02:43

Your issue is here:

var dayNum = d.getUTCDay() || 7;
d.setUTCDate(d.getUTCDate() + 4 - dayNum);

That code is for ISO weeks and adjusts Sunday's day number to 7 and sets the date to Thursday, where the week is Monday to Sunday.

You want weeks Sunday to Saturday and want the week of the Sunday, so use:

var dayNum = d.getUTCDay();
d.setUTCDate(d.getUTCDate() - dayNum);

which can be reduced to:

d.setUTCDate(d.getUTCDate() - d.getUTCDay());

 Date.prototype.getWeekNumber = function () {
        var d = new Date(Date.UTC(this.getFullYear(), this.getMonth(), this.getDate()));
        d.setUTCDate(d.getUTCDate() - d.getUTCDay());
        var yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
        return Math.ceil((((d - yearStart) / 86400000) + 1) / 7);
    };
    
var c = new Date(2017,7,26);
console.log(c.getWeekNumber() + ':' + c.toString());
var d = new Date(2017,7,27);
console.log(d.getWeekNumber() + ':' + d.toString());

This gives 53 as the answer iif I input 2018-01-01. That is incorrect @RobG — tharindu, Jul 25 '19 at 10:17
@tharindu—you're correct for ISO weeks, but the OP wants week numbers based on Sunday, not ISO's Thursday. 1 Jan 2018 was a Monday in the week that started on Sunday 31 Dec 2017. So, per the OP's requirement, it's in week 53 of 2017. PS. I've answered an ISO week number answer [here](https://stackoverflow.com/questions/6117814/get-week-of-year-in-javascript-like-in-php/6117889#6117889). — RobG, Jul 25 '19 at 10:52

score 1 · Answer 2 · answered Aug 28 '17 at 00:08

1

Your code d.setUTCDate(d.getUTCDate() + 4 - dayNum); is setting your date back to three days earlier. I think that's the reason you are getting previous week number. I checked removing the code and it worked fine.

answered Aug 28 '17 at 00:08

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Get week number with week starting from sunday

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