Daylight saving time
In roughly a third of the world's countries the clocks jump forward in spring and fall back in autumn. It moves an hour of daylight to the evening — and it changes how much time is left in the year.
What it is
Daylight saving time is the practice of advancing clocks, usually by one hour, to make better use of the longer daylight in summer by having darkness fall at a later clock time. In autumn the clocks return to standard time. The day of the spring change has 23 hours; the day of the autumn change has 25.
Where it came from
The idea was proposed by George Hudson in 1895 and independently by William Willett in 1907, who noticed on an early ride how many Londoners slept through the summer morning. The first nationwide implementations were by the German and Austro-Hungarian Empires, both starting on 30 April 1916; many countries followed.
Who uses it now
As of 2023 around 34 percent of the world's countries use it, mainly in Europe and North America. The European Union changes on the last Sunday in March and the last Sunday in October; the United States on the second Sunday in March and the first Sunday in November. Most of equatorial Africa has never used it — near the equator there is no summer daylight to shift.
Why the countdown gains an hour
Between 1 October and 1 January, a zone that falls back in autumn adds one hour to the clock. So from 1 October there are 92 days and 1 hour until midnight on 1 January in New York, but 92 days and 0 hours in UTC or in India, which do not change. The board computes the exact interval from the zone's own rules, so it shows the extra hour; a plain calendar count would miss it. This is the single most common reason the hours digit on the board differs from what people expect.
Standard time, not summer time, at New Year
In the Northern Hemisphere New Year falls in winter, so the offsets at midnight on 1 January are standard-time offsets: New York at UTC−5, London at UTC. In the Southern Hemisphere it is summer: Sydney at UTC+11 rather than +10, Auckland at UTC+13. The city-by-city table uses the offsets in force at that moment.
Sources
- Daylight saving time — Wikipedia (definition, Hudson 1895, Willett 1907, 30 April 1916, 34 percent, EU and US rules, equatorial Africa)