[FIXED] Why does LocalDate with custom DateTimeFormatter pattern incorrectly set the time with zone?

Issue

Given the following test I would have expected the time to be the same, but it's not.

@Test
public void canGetCorrectOffsetFromFormatterDateParse()  {

    Instant isoDateInstant = LocalDate.parse("2009-12-31",
            DateTimeFormatter.ISO_DATE)
            .atStartOfDay(ZoneId.systemDefault())
            .toInstant();
    System.out.println(isoDateInstant);
    assertThat(isoDateInstant).isEqualTo("2009-12-31T05:00:00Z");

    Instant customDateInstant = LocalDate.parse("03-31-2020",
            DateTimeFormatter.ofPattern("MM-dd-uuuu")
                    .withResolverStyle(ResolverStyle.STRICT))
            .atStartOfDay(ZoneId.systemDefault())
            .toInstant();
    System.out.println(customDateInstant);
    assertThat(customDateInstant).isEqualTo("2020-03-31T05:00:00Z");

}

This test fails with the assertion of the customeDateInstance

Expected :2020-03-31T05:00:00Z
Actual   :2020-03-31T04:00:00Z

Q: Why are the times that are being appended not the same, and how do I standardize the time portion going forward?

These tests were run in VA, USA, which is the America/New_York ZoneId.

Solution

Because of DST, America/New_York is GMT-5 in "winter" and GMT-4 in "summer".

In 2020:

• "winter" to "summer" was on Sunday, March 8, 2:00 am,
• "summer" to "winter" was on Sunday, November 1, 2:00 am.

As expected, your code then:

• gives 2020-03-05T05:00:00Z with 03-05-2020 ("winter"),
• gives 2020-03-15T04:00:00Z with 03-15-2020 ("summer").

Answered By - sp00m
Answer Checked By - David Goodson (JavaFixing Volunteer)