jQuery UI Datepicker showing incorrect week number

Question

When using the jQuery UI Datepicker and starting the week on a Sunday, the week numbers are incorrect. For example, 3rd Jan 2016 should be week 1 as all of the dates (3rd to 9th) are in the same year. But as you can see in the screenshot below, the UI shows it as week 53.

Here is the code to render the datepicker:

$("#datepicker" ).datepicker({
    showWeek: true,
    firstDay: 0
});

So nothing special, other than showing the week numbers and starting the week on Sunday instead of Monday (as per default).

Here is a fiddle of the issue: https://jsfiddle.net/vLqabmmz/

Answer 1

Due to the fact that this seems like a bug in jQuery UI. I'm posting the possible answer being the reporting of this as a bug to the jQuery UI team here: https://bugs.jqueryui.com/ticket/14907#ticket

Answer 2

i checked further and iso8601Week() method of jqueryUI is working fine, the problem is in its representation (the layout).

If you extract the value of the week you can see that the standard is followed and is fine. 3-rd is week 53 and after that 4 ... 7 (which is the first Thursday of the year and this is the week number 1). You can check with the code below.

  $(function() {
    $( "#datepicker" ).datepicker({
      showWeek: true,
      onSelect: function(dateText, inst) {
            $(this).val("'Week Number '" + $.datepicker.iso8601Week(new Date(dateText)));
      }
    });
  });

Answer 3

As seen from documentation for calculateWeek here:

This function uses the ISO 8601 definition of a week: weeks start on a Monday and the first week of the year contains January 4.

So comments are right qualifying this as a bug. While this is not resolved yet, you can overcome this easily by setting firstDay to 1 like this:

$("#datepicker" ).datepicker({
    showWeek: true,
    firstDay: 1
});

Week number would be correct, it is easier than rewrite calculateWeek, only difference is that week will start on Monday instead on Sunday. Note that default behavior sets firstDay to 0.

Answer 4

Here is a display of the jQuery calendar for the first week of 2017 - fully contained in a new year.
Yet, the week number is shown as 52 of year 2016 (wtf).

To me it looks like a bug to be corrected, otherwise there is no reason to display the week number.