I found strange behaviour when using Java's DateTimeFormatter on LocalDate for 30th and 31st of December.
LocalDate date1 = LocalDate.of(2019, Month.DECEMBER, 30);
System.out.println("date1: " + date1);
System.out.println("converted date1: " + DateTimeFormatter.ofPattern("YYYY-MM-dd").format(date1));
LocalDate date2 = LocalDate.of(2019, Month.JANUARY, 30);
System.out.println("date2: " + date2);
System.out.println("converted date2: " + DateTimeFormatter.ofPattern("YYYY-MM-dd").format(date2));
Output:
date1: 2019-12-30
converted date1: 2020-12-30
date2: 2019-01-30
converted date2: 2019-01-30
The first date (December 30) is converted with the next year, the second date (January 30) is converted with the correct year.
Am I missing something out, or is it a bug?
Y means "year of week-based year". It's not the same as y which means "year of era" as per docs.
This post explains the difference and purpose of Y:
A week year is a year where all the weeks in the year are whole weeks. This is specified by some standard (which I don't remember at the moment). Basically, this guarantees that a program working on a week's data will not transition between years. Unfortunately, this also means that the beginning of the year may not start on the first of January. What year a particular day belongs in depends on these rules, and of course, there are days where the year and the week year are different.
In your example 30st Dec '19 was a Monday, 31st Dec '19 was a Tuesday and 1st Jan '20 was Wednesday so "year of week-based year" for this three days is 2020.