When Do the Clocks Change and Why Do We Use Daylight Saving Time?

It's the bane of business meetings, dates, and early shift workers everywhere: Forget when the clocks go back, and you can find yourself severely out of time.

So when do the clocks change? How does it differ around the world? And what's the reason?

When should I change my clocks?

In the U.S., Canada, and nine other territories including Cuba and Bermuda, the clocks go backward by one hour on Sunday November 5, according to timeanddate.com. This marks the end of daylight saving time (DST.)

The exceptions are Arizona and Hawaii, which do not use DST.

At 2am in standard time where you are in the United States, the clock should jump back to 1am. That means there'll be more hours of sun at the start of the day, and an earlier sunrise and sunset.

In 56 territories around the world, including throughout Europe, the clocks will go backward on Sunday, October 29. In Syria and Jordan, it's October 27.

In Iran, the clocks turn back on September 27.

In Fiji and Tonga, daylight saving time actually begins on November 5, so the clocks go forward. It then ends on January 21 next year. In Chile, they don't go back until May 13 next year.

Similarly, in most parts of Australia and Paraguay, DST began on October 1, and clocks won't go backward until next year, on April 1 in Australia and March 25 in Paraguay.

And in Antarctica, New Zealand, and Samoa, DST began on September 24, and will end on April 1.

In the rest of the world, territories don't observe any daylight saving time.

When did we start changing the clocks?

The notion of shifting time around based on what the sun is doing is ancient. The Romans, for example, told the time in such a way as to continually adjust it around the behavior of the sun.

But DST as we understand it today is a relatively recent invention. In 1895, a New Zealander scientist called George Hudson presented a paper to the Wellington Philosophical Society that suggested a system of "seasonal time adjustment," according to an official biography.

Initially, he was ridiculed, but after New Zealand first successfully trialed daylight savings in 1927, Hudson was given a medal for his work in 1934. The haters will always come around eventually if your idea is good enough.

According to Timeanddate, a Canadian district called Port Arthur in Ontario turned their clocks forward in July 1908, the world's first actual DST period. Germany and Austria were the first whole countries to adopt the system, in 1916.

They were followed by the U.K., France, and others. The U.S. joined the bandwagon in 1918.

Why do we do it?

That first rush of countries was sparked by World War I. In Germany, the U.S. and elsewhere, the idea was adopted as a temporary emergency measure that was supposed to conserve fuel and give more hours of productive work.

Most countries dropped it once the fighting was over, but the Brits were an exception: They kept it on, and in 1925 officially made it permanent, calling it British Summer Time (BST).

In the U.S., DST came back with the return of war—it was taken up in 1942 as part of the battle against the Nazis. Then, up until 1966, states could decide whether to adopt it or not. Following that, the Uniform Time Act spread it across most states.

Proponents argue that using DST saves on energy costs, though the evidence is disputed. Meanwhile detractors point to the hassle involved, both on a personal level, and on a large scale in industry where it can make organizing suppliers and workers more difficult.

Why is it different around the world?

Some 172 countries just don't see the point: They've clearly never been convinced of the case for it.

Among the countries that do follow it, summertime is different in the Northern and Southern hemispheres, so that explains some of the biggest discrepancies.

Otherwise, it's simply a function of what governments decided in the past.