Daylight Saving Time Explained: Why Time Differences Change
Daylight saving time (DST) is the practice of moving clocks forward in spring and back in autumn. It is the main reason the time difference between two cities is not always the same number of hours.
What daylight saving time does
In spring, clocks are set forward by one hour, so sunrise and sunset happen one hour later by the clock. In autumn, clocks are set back and the region returns to standard time. The idea is to shift usable daylight toward the evening during the months when days are long. The total amount of daylight does not change; only the clock labels do.
Who uses it
Most of North America and Europe use daylight saving time, as do parts of Oceania, South America and the Middle East. Most of Asia and Africa do not, and neither do many countries close to the equator, where day length changes little through the year. Even inside countries that use it there are exceptions: Hawaii and most of Arizona stay on standard time all year.
Rules also change. Brazil ended daylight saving time in 2019, Mexico ended it in 2022, and Egypt reintroduced it in 2023. This is why time zone databases are updated several times a year and why it helps to keep your browser or device up to date.
When the clocks change
- United States and Canada: clocks go forward on the second Sunday in March and back on the first Sunday in November.
- European Union and United Kingdom: clocks go forward on the last Sunday in March and back on the last Sunday in October.
- Australia (states that use DST, such as New South Wales and Victoria): clocks go forward on the first Sunday in October and back on the first Sunday in April.
- New Zealand: clocks go forward on the last Sunday in September and back on the first Sunday in April.
Because the southern hemisphere has summer when the north has winter, "spring forward" happens in September or October there.
Why gaps between cities shift
When two places change their clocks on different dates, the time difference between them is temporarily different. The classic example is New York and London. Normally London is five hours ahead of New York. But the United States moves its clocks in early March and the United Kingdom does so in late March, so for roughly three weeks the gap is only four hours. The same thing happens in the autumn between the end of October and the first Sunday in November.
Places that never change their clocks, such as Tokyo, Singapore or Dubai, see their gap to DST regions change by one hour twice a year. For example, Tokyo is 14 hours ahead of New York in winter and 13 hours ahead in summer.
Practical tips
- When you schedule across regions, always write the date, the time and the city, not just a number of hours.
- Be extra careful about meetings in the weeks around March and October/November.
- For recurring meetings, check whether your calendar keeps the meeting at a fixed local time in one city or a fixed UTC time.
Check a specific date: Time Compare applies the daylight saving rules for the exact date you choose.
Related guides
How to Schedule Meetings Across Time Zones
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Time Zones in the United States: Zones, DST and Exceptions
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