Why does the thirteen.org date library return dates in the same year for a 53-week year (for week 53-CodePudding

Using the https://www.threeten.org/threeten-extra library.

When computing the first Sunday of the month, given week number and year, I noticed that if I used 2018 and week 53, I get as the result the first Sunday 2018-12-02.

Yet, 2018 is not a 53-week year so I am wondering if I'm doing something wrong or perhaps there's something special about 2018? I didn't see that the week start date or other 53-week conditions were met, but I could have missed something.

In other tests like 2011 (also not a and week 53-week year), the date logic wraps the result as expected to 2012-01-01 (the first Sunday in the first week of the next year).

Years 2020, 2015, and 2019 (all 53-week years) display the result value as expected in December of the respective year.

Here is the code:

YearWeek yw = YearWeek.of(year, weekNumber);
//Get the date of the first day of that week.
LocalDate ld = yw.atDay( DayOfWeek.MONDAY ) ;
// Use a TemporalAdjuster to get nth day-of-week of that month date’s month.
TemporalAdjuster ta = TemporalAdjusters.dayOfWeekInMonth( 1 , DayOfWeek.SUNDAY ) ;
//  TemporalAdjuster ta = TemporalAdjusters.firstDayOfMonth();
LocalDate firstSunday = ld.with( ta ) ;
return firstSunday.toString();

Suggestions?

CodePudding user response：

This is what YearWeek.of does:

Obtains an instance of YearWeek from a week-based-year and week.

If the week is 53 and the year does not have 53 weeks, week one of the following year is selected.

Therefore, for (2018, 53), it selects the first week of 2019, because 2018 only has 52 weeks. This is the week starting from 2018-12-31 and ending at 2019-01-06.

You then get the Monday of this week, which is 2018-12-31, and adjusts this date to the first Sunday of the month. "The month" here being December, you get 2018-12-02 as a result.

Similarly, for (2011, 53), you get the first week of 2012 because 2011 only has 52 weeks. But unlike 2019, this is the week starting from 2012-01-02 and ending at 2012-01-08, so there is no problem.

So what's "special" about 2018? Its last week ends before the last day of the year, I suppose? This causes the first week of the next year to start in December.