At 2am on Sunday 12 March the clocks skipped ahead one hour to 3am, as the country left standard time behind. On 26 March, the UK will follow suit, entering British Summer Time, which is one hour ahead of GMT.
The century-old practice (DST turned 100 in 2016) is supposed to redistribute daylight to more useful hours.
The time shift – which will cost first millions of Americans, and then millions of Britons, an hour of sleep – is billed as bringing brighter evenings, meaning no more commuting home in the dark.
But critics say it is an oppressive and disorientating practice designed to better control workers.
Here’s everything you need to know:
When was DST introduced and why?
Benjamin Franklin, the American inventor and politician, first suggested the idea in an essay as a way for people to use fewer candles by making the most of early morning light.
In England, William Willet introduced the idea of a British Summer Time in 1907, and spent much of the rest of his life trying to achieve it before dying.
Germany adopted the idea first in 1916 and Britain — which was then fighting the country at the time — followed suit to help the war economy a year later.
In 1918, the US followed Europe’s example.
Who has DST?
Almost all countries in Europe now have DST, except for Belarus, Iceland, Georgia and Armenia.
Russia does not use it, even though the Norwegian archipelago of Svalbard, between mainland Norway and the North Pole, has it – as do Turkey, Syria, Jordan and Israel in the Middle East.
Mongolia switched to DST recently, after re-introducing the idea in 2015.
In Africa, Morocco and the Western Sahara changed their clocks last year, as well as Namibia at the south of the continent.
Most of north America has DST, with the exception of the states of Arizona and Hawaii – although each time zone across the US starts DST at different times.
But most of south America does not, with the exception of parts of southern Brazil.
Big Ben: Behind the clock face
The south east of Australia does use it, as does New Zealand.
But China does not plan to turn the clocks forward.
Countries sometimes announce just days or weeks ahead that they will take part in the time change, so the list is not exhaustive.
Was it always one hour forwards or back?
No — at points throughout history there have been 30 minute changes, two hour changes and even 20 and 40 minute changes.
Why is DST controversial?
Critics of DST claim darker mornings put children at risk walking to school.
On a more ideological level, some people see controlling time as an act of state oppression.
Pushed as a progressive, civilising tactic, critics have said the introduction of DST was originally linked to imperialism in addition to being part of a push by bosses to get as much labour out of workers as possible.
But fans argue setting the looks forward saves energy, boosts tourism and encourages more people to exercise outdoors.