Time difference between LON and SYD is 9 hours with daylight savings - datetime

Question

I am confused about time difference between LON and APAC regions. With UK day light saving, time difference between London and Singapore is 7 hours and without daylight saving, it is 8 hours.
But time difference in Sydney is 9 hours with day light saving and 11 hours without day light saving.
So my question is why there is extra one hour difference between LON and SYD
I have tested this with below code with system time zone as London
#Test
public void datetimetest(){
LocalDateTime dt1 = LocalDateTime.of(2019, 3, 19, 11, 0);
LocalDateTime dt2 = LocalDateTime.of(2019, 4, 19, 11, 0);
ZonedDateTime zonedDateTimeLon1 = dt1
.atZone(systemDefault())
.withZoneSameInstant(ZoneId.of("Europe/London"));
ZonedDateTime zonedDateTimeLon2 = dt2
.atZone(systemDefault())
.withZoneSameInstant(ZoneId.of("Europe/London"));
printZonedDateTime(zonedDateTimeLon1, "Europe/London");
printZonedDateTime(zonedDateTimeLon1, "Australia/Sydney");
printZonedDateTime(zonedDateTimeLon1, "Asia/Singapore");
printZonedDateTime(zonedDateTimeLon2, "Europe/London");
printZonedDateTime(zonedDateTimeLon2, "Australia/Sydney");
printZonedDateTime(zonedDateTimeLon2, "Asia/Singapore");
}
private static void printZonedDateTime(ZonedDateTime zonedDateTimeLon, String timeZone) {
LocalDateTime dateTime = zonedDateTimeLon
.withZoneSameInstant(ZoneId.of(timeZone))
.toLocalDateTime();
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
System.out.println(timeZone + "= " + dateTimeFormatter.format(dateTime));
}
Result:
Europe/London= 2019-03-19 11:00:00
Australia/Sydney= 2019-03-19 22:00:00
Asia/Singapore= 2019-03-19 19:00:00
Europe/London= 2019-04-19 11:00:00
Australia/Sydney= 2019-04-19 20:00:00
Asia/Singapore= 2019-04-19 18:00:00
As shown in results, time difference in April is 9 hours for Sydney. Can anyone please explain this

Answer 1

Today (March 19) Sydney is still using summer time (DST). The summer on the southern hemisphere coincides with the winter on the northern. So it’s ending around this time. So while standard time in Sydney is at offset +10:00, they are at +11:00 now. London is currently on standard time, +00:00. So the difference is 11 hours as you have observed.
Summer time begins in London (and the EU) on March 31, bringing London on offset +01:00, in turn reducing the difference to 10 hours.
Summer time ends in Sydney on April 7. They return to their standard offset of +10:00, further reducing the difference between London and Sydney to 9 hours, the difference that you observed for April 19.
Or in code:
ZoneId london = ZoneId.of("Europe/London");
ZoneId sydney = ZoneId.of("Australia/Sydney");
Instant instMarch = Instant.parse("2019-03-19T00:00:00Z");
Instant instApril = Instant.parse("2019-04-19T00:00:00Z");
System.out.println(instMarch.atZone(london));
System.out.println(instMarch.atZone(sydney));
System.out.println(instApril.atZone(london));
System.out.println(instApril.atZone(sydney));
The output is:
2019-03-19T00:00Z[Europe/London]
2019-03-19T11:00+11:00[Australia/Sydney]
2019-04-19T01:00+01:00[Europe/London]
2019-04-19T10:00+10:00[Australia/Sydney]
Please note that the current offsets are printed (Z in the first output line means offset zero).
Singapore in turn does not use summer time (at least not in 2019), so here the difference is only reduced from 8 to 7 hours when summer time begins in London.