Formatting Java Instant to "YYYYMMdd" adds an extra year?

Question

It's been a long day and my brain is tired so maybe I'm completely missing something, but how come when I run this line,

System.out.println(
  DateTimeFormatter.ofPattern("YYYYMMdd")
    .withZone(ZoneId.of("UTC"))
    .format(Instant.parse("2020-12-31T08:00:00Z"))
  )
)

I get 20211231 instead of 20201231? Where does the extra year come from?

score 7 · Accepted Answer · answered Jan 15 '20 at 22:09

7

You want lowercase y -

        System.out.println(
            DateTimeFormatter.ofPattern("yyyyMMdd")
                    .withZone(ZoneId.of("UTC"))
                    .format(Instant.parse("2020-12-31T08:00:00Z"))
    );

answered Jan 15 '20 at 22:09

Kim

150
7

score 5 · Answer 2 · answered Jan 15 '20 at 22:11

According to the docs of DateTimeFormatter, Y is the week-based year. That means that the year number of the week in which the date falls, is returned.

The fact is that 31st of December, 2020, is actually week 1 of 2021, hence 2021 is returned instead of 2020.

You probably want to use yyyy instead.

score 4 · Answer 3 · answered Jan 15 '20 at 22:11

The capital Y represents Week based year as per official documentation of DateTimeFormatter. More about how it calculates over here.

This works fine if you use date formatter with smallcase y as yyyyMMdd.

System.out.println(DateTimeFormatter.ofPattern("yyyyMMdd").withZone(ZoneId.of("UTC"))
            .format(Instant.parse("2020-12-31T08:00:00Z")));

Output:

20201231

Formatting Java Instant to "YYYYMMdd" adds an extra year?

3 Answers3