Incorrect week numbers 2025/2026

[chuckn0rris]

Forum post

Hello,

We use several custom presets where we show the weeknumbers. Most of our customer prefer weeknumbers over dates. According to your documentation if we use dateFormat: "WW" this should show the ISO weeknumbers. However around the end of 2025, start of 2026 this is not the case.

I reproduced this in an example project, React-vite advanced in v6.1.6 of the Gantt. I added the following code to GanttProps:

  presets:[
        {
            id: "weekDateAndMonth",
            name: "Weeks 3",
            rowHeight: 24,
            tickWidth: 30,
            tickHeight: 40,
            displayDateFormat: "ll",
            shiftUnit: "week",
            shiftIncrement: 1,
            defaultSpan: 10,
            timeResolution: {
            unit: "day",
            increment: 1,
            },
            headers: [
            {
                increment: 1,
                unit: "month",
                dateFormat: "YYYY MMMM",
            },
            {
                increment: 1,
                unit: "week",
                dateFormat: "WW",
            },
            ],
            mainHeaderLevel: 1,
            columnLinesFor: 1,
        },
    ],
    viewPreset: "weekDateAndMonth",

This sets the viewpreset to one that shows weeknumbers. If I then navigate to the end of 2025, I see a week 53.

However according to the ISO system(and also for example Outlook), it should go from week 52 to week 1. According to the ISO rule the year of the weeknumber is based on which year thursday falls in. So with thursday being on 1st January 2026, this is week 1. https://en.wikipedia.org/wiki/ISO_week_date

Which should result in:

So I think this is a bug. Is there an easy way to implement a custom render function to correct this?

[chuckn0rris]

Related to #10991