How does Java “week year” really work?

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It’s simple: December 27 2015 is day 1 of week 1 of week-year 2016 (and December 27 2026 is day 1 of week 1 of week-year 2027). This can be verified by adding these lines:

SimpleDateFormat odf = new SimpleDateFormat("YYYY-ww-u");
System.out.println(odf.format(d1));
System.out.println(odf.format(d2));
System.out.println(odf.format(d3));

If a SimpleDateFormat outputs a date it can use all fields: year, month, day, day of week, week of month, week in year, week-year etc.

On parsing, SimpleDateFormat expects a matching set of values: either day, month, year or day of week, week in year, week-year. Since you supplied a week-year but did not supply day of week and week in year, those to values have been assumed as 1.

The actual values depend on your locale:

  • which week of a year is week 1
  • which day is the first day of the week

(see https://docs.oracle.com/javase/7/docs/api/java/util/GregorianCalendar.html#week_and_year)

On my system (using de-ch locale, with “EEE MMM dd HH:mm:ss zzz yyyy – YYYY-ww-u” as format) I get

Mo Jan 04 00:00:00 MEZ 2016 - 2016-01-1
Mo Jan 04 00:00:00 MEZ 2016 - 2016-01-1
Mo Jan 04 00:00:00 MEZ 2027 - 2027-01-1

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