About one third of all countries observe Daylight Saving Time (DST), also called ‘summer time’. This is a seasonal change of the clock: moving one hour forward to get longer daylight in the evenings of the summer months. Or in other words your clock: springs forward (1 hour ahead in spring time) and falls back (return to standard time zone in the fall or autumn time of the year).
Start and end dates of daylight saving are government decisions. This resulted in fifteen different DST periods, which can change every year.
On top of the time zone differences, daylight saving time makes it even harder to correctly calculate what time it is in places in another country or continent.
Four cities: two with daylight saving
London, England, observes the European summer time from the last Sunday in March until the Saturday before the last Sunday in October.
Lagos has no daylight saving, because Nigeria never used it.
Australia observes DST, but not the state of Queensland with Brisbane as its capital.
Sydney, Australia is the capital of New South Walies, one of the states that does observe daylight saving: from the first Sunday in October until the Saturday before the first Sunday in April. This is a totally different period than in Europe, because the summer months are different on the Southen Hemisphere.
Let’s see what this means for the time in these four cities throughout the year.
London – Sydney: 11 hours time difference
End of January means winter time in London but still summer time in Sydney. Despite that both Brisbane and Sydney are on the east coast of Australia, Sydney is one hour ahead due to its daylight saving time.
London – Sydney: 10 hours time difference
Now London has summer time, which started on the last Sunday in March. Because the clocks changed in London, the time difference with Lagos disappeared.
This 1st of April is not a Sunday, so Sydney still has a few days of summer time to go.
Overall, the time difference between Sydney and London has decreased with 1 hour.
London – Sydney: 9 hours time difference
At the end of April, winter time has started in Sydney but in London it is still summer time, until October. Moving the clock back one hour from summer to winter time, Sydney has decreased its time difference with London by another hour.
Now Brisbane and Sydney have the same time, during their winter months.
Time differences not only due to time zones
You see that these four cities can have two, three or four different local times, depending on their time zone plus summer time, the latter depending on the date. This make calculating the local time in another country or state very complicated. To save time and embarrasment, you better leave this to a reliable website or app.
Apps can help with time zone and daylight saving math
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Apart from acting as a world clock, Horzono also visualizes how time zones relate and overlap: see the above screen shots. You can compare up to 10 cities for free.
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