Does Java 8's ZonedDateTime take into account daylight savings when converting?

Question

I have the following code below:

ZonedDateTime zonedDateTime = ZonedDateTime.ofInstant(
            this.start.toLocalDateTime(), ZoneOffset.UTC,
            ZoneOffset.systemDefault());

The this.start is from a java.sql.Timestamp. I am explicitly converting from UTC here to the system's local offset, will this be smarty enough to take into account daylight savings?

Answer 1

ZonedDateTime takes daylight savings into account. That's what differs it from LocalDateTime. Well, that and timezone information.

Answer 2

The accepted answer lacks an example. It also compares a ZonedDateTime with a LocalDateTime in the context of DST which does not make sense as a LocalDateTime lacks a time zone. In the context of DST, the comparison should be between a ZonedDateTime and an OffsetDateTime. While a ZonedDateTime, automatically changes the zone offset for a ZoneId according to the DST, an OffsetDateTime is used with a fixed zone offset.

Demo

public class Main {
    public static void main(String[] args) {
        ZoneId zoneId = ZoneId.of("America/New_York");

        // A sample ZonedDateTime during DST
        ZonedDateTime zdtDuringDST = ZonedDateTime.of(
                LocalDate.of(2024, 5, 20),
                LocalTime.MIN,
                zoneId
        );

        // A sample ZonedDateTime outside DST
        ZonedDateTime zdtOutsideDST = ZonedDateTime.of(
                LocalDate.of(2024, 12, 20),
                LocalTime.MIN,
                zoneId
        );

        System.out.println(zdtDuringDST);
        System.out.println(zdtOutsideDST);
    }
}

Output:

2024-05-20T00:00-04:00[America/New_York]
2024-12-20T00:00-05:00[America/New_York]

Online Demo

As you can see in the results, the ZonedDateTime automatically set the applicable offsets, -04:00 and -05:00 based on the DST. On the other hand, you use an OffsetDateTime with a fixed zone offset e.g. the zone offset, +01:00 in the below OffsetDateTime will remain fixed irrespective of change in date and time:

ZoneOffset zoneOffset = ZoneOffset.ofHoursMinutes(1, 0);

OffsetDateTime.of(
        LocalDate.of(2024, 5, 20),
        LocalTime.MIN,
        zoneOffset
);

Learn more about the modern Date-Time API from Trail: Date Time.