What does DateTimeFormatter parse LocalDate.of(0,1,5) to "010105"?

Question

When using DateTimeFormatter of format "yyMMdd", strange things happen... LocaleDate.of(0,1,5) is formatted to "010105", but "000105" is correctly parsed to LocalDate.of(2015, 1, 5). Why is that??

For testing:

@Test
public void testYear2000() {
    LocalDate localDate = LocalDate.of(0, 1, 5);
    String format = DateTimeFormatter.ofPattern("yyMMdd").format(localDate);

    assertThat(format, is("000105")); // fail! it is "010105"

    LocalDate parse = LocalDate.parse("000105", DateTimeFormatter.ofPattern("yyMMdd"));

    assertThat(parse, is(LocalDate.of(2000, 1, 5))); //pass
}

Answer 1

I add the supplementary explanation for the correct answer of @Lachezar Balev. Printing the year 0 (more than two thousand years ago) by the pattern yy means: Use the two-digit-form of year-of-era equivalent to proleptic gregorian year 0 which is 1 BCE. Therefore you see the year 1 in formatted output 010105.

If you had used the pattern uuMMdd (u=proleptic gregorian year) then the output would indeed match your expectation 000105 but is still fundementally wrong due to wrong input.

Answer 2

Your confusion comes from the fact that here:

LocalDate.of(0, 1, 5);

you have specified year zero, which is millenniums before the year of 2000:

LocalDate.of(2000, 1, 5);

The last will output 000105.

Update: @Meno Hochschild added further explanations and I would also link this nice question for reference.