Earliest and Latest ZonedDateTimes on Earth from an UTC reference time

April 8, 2022

I have a reference ZonedDateTime in UTC:

var ref = ZonedDateTime.parse("2022-01-01T14:00Z");

We must find the earliest and latest times on Earth based on the most extreme known TimeZones.

That would be:

2022-01-02T04:00+14
2022-01-01T03:00-11

What is the most reliable way to achieve this?

I’m thinking about OffsetDateTime, but I’m not sure it will handle daylight times correctly. Are there constants for the minimum and maximum offsets and/or TZ?

>Solution :

You can do a stream on ZoneId.getAvailableZoneIds().

var instant = Instant.parse("2022-01-01T14:00:00Z");
var minMax = ZoneId.getAvailableZoneIds().stream()
    .mapToInt(x -> ZoneId.of(x).getRules().getOffset(instant).getTotalSeconds())
    .summaryStatistics();

var latestLocalTime = instant.atOffset(ZoneOffset.ofTotalSeconds(minMax.getMax())).toLocalDateTime();
var earliestLocalTime = instant.atOffset(ZoneOffset.ofTotalSeconds(minMax.getMin())).toLocalDateTime();
System.out.println(latestLocalTime);
System.out.println(earliestLocalTime);

Notice that this prints out, in Java 17:

2022-01-02T04:00
2022-01-01T02:00

The earliest time is different from what you expect. I looked into what this timezone is:

System.out.println(ZoneId.getAvailableZoneIds().stream()
    .map(ZoneId::of).min(Comparator.comparingInt(
        x -> x.getRules().getOffset(instant).getTotalSeconds()
    )));

It is apparently the timezone Etc/GMT+12. Despite its identifier, this timezone actually has an offset of -12 hours, because the Etc/GMT identifiers use the opposite signs. Anyway, UTC-12 is technically observed in Baker Island and Howland Island, according to Wikipedia.