weeksInWeekYear returns wrong results

Describe the bug The year 2021 has 52 ISO Weeks (compare with Google Calendar), but luxon returns 53 for the year 2021. This must be wrong, because Week 52 goes from 27.12.21 to 02.01.22. After this the first week of 2022 must follow, cause it always contains 4th january (see ISO 8601 for reference)

To Reproduce

console.log(DateTime.fromISO("2021").weeksInWeekYear) // returns 53

Actual vs Expected behavior Should return right number of weeks.

Workaround

function weeksInYear(year) {
 return DateTime.fromISO(year + "-12-28").weekNumber;
}

Desktop (please complete the following information):

• OS: [e.g. iOS] Windows
• Browser [e.g. Chrome 84, safari 14.0] Chrome 96
• Luxon version [e.g. 1.25.0] 2.2.1
• Your timezone [e.g. “America/New_York”] Europe/Berlin

[icambron]

I think I can guess the confusion. The “ISO” in “fromISO” doesn’t refer to the ISO week calendar; it refers to the ISO 8601 string format. When parsing a just alone year, that refers to the Gregorian year. Does that help?

[Edit] Try fromObject({weekYear: 2021}).weeksInWeekYear()

[icambron]

DateTimes are specific instants in time. fromISO returns such an instant. That instant is in the 2021 Gregorian year but not the 2021 ISO year. Thus weeksInWeekYear is returning the number of weeks in the iso week year your datetime is actually in