Check the time in London and Sydney one week and the gap is 11 hours. The next week it is 10. A month later, 9. This happens because countries do not all switch clocks on the same day. The gap shifts for two reasons: DST start and end dates differ, and the two cities sit in opposite halves of the globe. These moving gaps can trip up travellers, remote workers, and anyone scheduling calls. Here is how the mechanics work, with examples you can use to predict the gap at any time of year.
The Core Reason Different DST Calendars
Daylight saving time is not observed everywhere. Even among places that use it, the start and end dates vary. The United States begins DST on the second Sunday in March and ends on the first Sunday in November, both at 2:00 local time. The European Union and the United Kingdom start on the last Sunday in March and end on the last Sunday in October, switching at 01:00 UTC. In the south, southern Australia (New South Wales, Victoria, Australian Capital Territory, Tasmania, and South Australia) begins DST on the first Sunday in October and ends on the first Sunday in April. Because these dates do not align, the time difference between two cities can change several times a year.
Worked Pair Sydney and London
Sydney and London are a classic case of how opposite halves of the globe and different DST calendars create a shifting gap. Sydney uses Australian Eastern Standard Time (AEST, UTC+10) during standard time and Australian Eastern Daylight Time (AEDT, UTC+11) during DST. London uses Greenwich Mean Time (GMT, UTC+0) during standard time and British Summer Time (BST, UTC+1) during DST.
Northern Winter (November to March)
During the northern winter, London is on GMT (UTC+0) and Sydney is on AEDT (UTC+11) because it is summer in the south. The gap is 11 hours, with Sydney ahead.
Northern Summer (April to October)
In the northern summer, London moves to BST (UTC+1) while Sydney moves to AEST (UTC+10) because it is winter in the south. The gap narrows to 9 hours.
Transition Weeks
For a few weeks each year, one city is on DST while the other is not. In late March, London switches to BST while Sydney is still on AEDT, creating a 10 hour gap. In early April, Sydney switches to AEST while London is still on BST, again producing a 10 hour gap. In October, the reverse happens: Sydney switches to AEDT while London is still on BST, giving a 10 hour gap. The gap is 11 hours in the northern winter, 9 hours in the northern summer, and 10 hours during the transition weeks.
How Opposite Hemispheres Affect the Gap
When two cities sit on opposite sides of the equator, their DST periods are reversed. Northern summer DST (March to October in Europe, March to November in the US) overlaps with the southern winter, when many places in the south are on standard time. This means the gap between a northern city and a southern city can shrink or grow by 2 hours over the course of a year, as seen with Sydney and London.
If both cities are in the same half of the world, the gap is more stable. New York (US DST) and London (EU DST) have a 5 hour gap for most of the year. For a few weeks in March and October the gap shifts to 4 hours because their DST start and end dates differ by a week or two.
DST Mismatch Weeks in Detail
The mismatch weeks happen because the US, EU/UK, and southern Australia all change clocks on different dates. Here are the transition periods for Sydney and London:
- Late March: London switches to BST on the last Sunday of March. Sydney is still on AEDT (southern summer DST). Gap: 10 hours.
- Early April: Sydney switches to AEST on the first Sunday of April. London is on BST. Gap: 10 hours.
- Late October: London switches to GMT on the last Sunday of October. Sydney is still on AEST. Gap: 11 hours.
- Early October: Sydney switches to AEDT on the first Sunday of October. London is still on BST. Gap: 10 hours.
These mismatch periods last one to two weeks each. The gap is not constant for about a month total each year.
Cities That Never Change
Some time zones do not observe DST at all. Japan is always UTC+9. India is always UTC+5:30. Pair a city like Tokyo (no DST) with London (DST) and the time difference shifts twice a year. In the northern winter, Tokyo is 9 hours ahead of London. In the northern summer, London moves to BST (UTC+1), so Tokyo is 8 hours ahead. The same effect occurs with any non DST zone paired with a DST zone.
Within Australia, Queensland does not use DST. During the southern summer, Sydney (AEDT) is 1 hour ahead of Brisbane (AEST). In the southern winter, both are on AEST, so the gap disappears. This is another example of how mismatched clock changes create shifting differences even within the same country.
How to Predict the Gap Yourself
Figure out the time difference between two cities at any date by following these steps:
- Find the standard time offset of each city (Sydney is UTC+10, London is UTC+0).
- Check if each city is currently observing DST. Use the known start and end dates for that region.
- Add 1 hour to the offset of any city that is currently on DST.
- Subtract the smaller offset from the larger offset to get the gap.
For Sydney and London in mid January: Sydney is on AEDT (UTC+11), London is on GMT (UTC+0). 11 minus 0 equals 11 hours. In mid July: Sydney is on AEST (UTC+10), London is on BST (UTC+1). 10 minus 1 equals 9 hours. The same method works for any pair.
Why This Matters for Scheduling
Schedule a recurring meeting or book a flight between two cities that use DST and you must account for the changing gap. A 9 am call in Sydney might be 10 pm the previous day in London during the northern summer, 11 pm during the northern winter, and 10 pm in the transition weeks. Before confirming a time, check the current DST status of both locations. The same rule applies when you plan travel between Sydney and London.