Few developers are aware of the difference between Y for "Week year" and y for Year when formatting and parsing a date
with SimpleDateFormat or DateTimeFormatter. That’s likely because for most dates, Week year and Year are the same, so
testing at any time other than the first or last week of the year will yield the same value for both y and Y. But in the
last week of December and the first week of January, you may get unexpected results.
According to the Javadoc:
A week year is in sync with a WEEK_OF_YEAR cycle. All weeks between the first and last weeks (inclusive) have the same week year value.
Therefore, the first and last days of a week year may have different calendar year values.
For example, January 1, 1998 is a Thursday. If getFirstDayOfWeek() is MONDAY and getMinimalDaysInFirstWeek() is 4 (ISO 8601 standard compatible
setting), then week 1 of 1998 starts on December 29, 1997, and ends on January 4, 1998. The week year is 1998 for the last three days of calendar
year 1997. If, however, getFirstDayOfWeek() is SUNDAY, then week 1 of 1998 starts on January 4, 1998, and ends on January 10, 1998; the first three
days of 1998 then are part of week 53 of 1997 and their week year is 1997.
Noncompliant code example
Date date = new SimpleDateFormat("yyyy/MM/dd").parse("2015/12/31");
String result = new SimpleDateFormat("YYYY/MM/dd").format(date); //Noncompliant; yields '2016/12/31'
result = DateTimeFormatter.ofPattern("YYYY/MM/dd").format(date); //Noncompliant; yields '2016/12/31'
Compliant solution
Date date = new SimpleDateFormat("yyyy/MM/dd").parse("2015/12/31");
String result = new SimpleDateFormat("yyyy/MM/dd").format(date); //Yields '2015/12/31' as expected
result = DateTimeFormatter.ofPattern("yyyy/MM/dd").format(date); //Yields '2015/12/31' as expected
Exceptions
Date date = new SimpleDateFormat("yyyy/MM/dd").parse("2015/12/31");
String result = new SimpleDateFormat("YYYY-ww").format(date); //compliant, 'Week year' is used along with 'Week of year'. result = '2016-01'
DateTimeFormatter.ofPattern("YYYY-ww").format(date); //compliant; yields '2016-01' as expected
