Southern Hemisphere Daylight Saving: Why Your Meeting Time Drifts Twice as Much

Everyone who schedules across timezones eventually learns that clocks change twice a year. Far fewer know that when the two cities are in different hemispheres, the gap between them changes by two hours across the year rather than one — and that for a few weeks each year, both sides are on summer time simultaneously.

If you work between Europe or North America and Australia, New Zealand or Chile, this is the mechanism that quietly breaks your recurring meetings.

Why the Gap Moves Twice As Much

Daylight saving moves a clock forward in the local spring and back in the local autumn. Spring in Sydney is September and October; spring in London is March and April. The two hemispheres are therefore always doing the opposite thing.

When both cities observe DST but in opposite seasons, the effects add rather than cancel:

Period	London	Sydney	Gap
Northern summer (Jul)	UTC+1 (BST)	UTC+10 (AEST)	9 hours
Overlap weeks (early Oct)	UTC+1 (BST)	UTC+11 (AEDT)	10 hours
Northern winter (Jan)	UTC+0 (GMT)	UTC+11 (AEDT)	11 hours
Overlap weeks (Apr)	UTC+0 (GMT)	UTC+10 (AEST)	10 hours

The gap ranges from nine to eleven hours over the year. A meeting fixed at 8:00 AM London lands at 5:00 PM Sydney in July and 7:00 PM Sydney in January — the difference between a normal end of day and an intrusion into someone's evening, with nobody having changed anything.

The Weeks When Both Are on Summer Time

Because the transition dates do not align, there are two periods each year when the hemispheres are briefly out of their usual relationship.

Early October. Eastern Australia moves to daylight time on the first Sunday in October. Europe does not leave summer time until the last Sunday in October. For roughly three weeks, both are on summer time at once and the gap sits at an intermediate value it holds at no other point in the year.

Late March into April. Europe enters summer time on the last Sunday in March; eastern Australia leaves daylight time on the first Sunday in April. Another short window with a gap that appears at no other time.

What this means in practice: a recurring north–south meeting pinned to local time on either side will shift four times a year, not twice — once for each hemisphere's spring and autumn transition. Anyone who has memorised the offset will be wrong during all four.

New Zealand Runs a Different Schedule Again

New Zealand's daylight saving season is longer than eastern Australia's at both ends. It begins on the last Sunday in September, ahead of Australia's first Sunday in October, and ends on the first Sunday in April, in most years the same weekend Australia ends but not always aligned.

The practical effect is on trans-Tasman scheduling. Sydney and Auckland are normally two hours apart, but for a week or so each September they are three, because Auckland has moved and Sydney has not. Teams with people in both countries routinely miss this — it is the classic trans-Tasman recurring-meeting failure.

Period	Sydney	Auckland	Gap
Southern winter	UTC+10	UTC+12	2 hours
Late Sep – early Oct	UTC+10	UTC+13	3 hours
Southern summer	UTC+11	UTC+13	2 hours

Australia Is Not One Timezone

A further complication specific to Australia: the country spans three main zones, and they do not all observe daylight saving.

New South Wales, Victoria, ACT, Tasmania — UTC+10, moving to UTC+11 in daylight time.
Queensland — UTC+10, and does not observe daylight saving. Aligned with Sydney for half the year and an hour behind for the other half.
South Australia — UTC+9:30, moving to UTC+10:30. A half-hour zone that also observes DST.
Northern Territory — UTC+9:30, no daylight saving.
Western Australia — UTC+8, no daylight saving. Three hours behind Sydney in the southern summer, two in winter.

"Australian time" in a meeting invite is therefore ambiguous across as much as three hours. Brisbane and Sydney are the same for part of the year and different for the rest, which makes Queensland the most common source of intra-Australian confusion.

South America Adds Its Own Pattern

The southern hemisphere is not uniform in this either. Chile still observes daylight saving on a southern schedule, moving forward in September and back in April. Brazil abolished it entirely in 2019 and now holds a fixed offset year-round. Argentina has not observed it since 2009.

So a Santiago–London relationship shifts on four occasions a year, while a São Paulo–London relationship shifts on two — both driven entirely by London. Two cities a few hundred kilometres apart in South America behave completely differently against the same European counterpart.

How to Schedule Through It

Book in UTC and let the calendar convert. This is the entire solution to the recurring-meeting problem, and it works because UTC never shifts. If your calendar supports setting a meeting's timezone explicitly, set it to UTC rather than to whichever local zone the organiser happens to be in.

Expect four review points a year, not two. Put the four transition dates relevant to your specific pair in a shared calendar. Any meeting whose local time matters — anything near the start or end of someone's day — should be checked at each one.

Choose slots with headroom. If a meeting is scheduled at 5:00 PM Sydney and the gap can move an hour in the wrong direction, it becomes a 6:00 PM meeting for months. Picking a time an hour clear of someone's boundary means the seasonal drift does not push it over.

Name the city, always. Not "Australian time", not "AEST" — Sydney, Brisbane or Perth. The abbreviation is ambiguous and the country spans three hours.

See the current gap, not the remembered one

These pages recompute offsets and transition dates from the timezone database at build time, so they show what is true now rather than what was true when someone wrote it down. See London–Sydney, London–Auckland, Sydney–Auckland, or add your own cities to the grid.

◎ Open the timezone grid