June 21st is usually seen as the longest day of the year, but in 2015, it's June 30th that technically holds that distinction. And it could be a problem.

With the advent of ever-more-accurate clocks, keeping good time is becoming surprisingly complicated.

When Henri met Albert the stars didn’t quite align; nor did their clocks. Jimena Canales, historian of science, tells Joe Gelonesi about her discovery of an explosive 20th century debate that changed our view of time and destroyed a reputation.

“Is there anything we know more intimately than the fleetingness of time, the transience of each and every moment?”

300 years ago, on the 3rd of May 1715, a rare solar eclipse occurred over England. It was an opportunity too good to miss for those promoting new astronomical theories – and their own careers.

It wouldn't gain or lose a single second in 15 billion years.

The pendulum clock of Longitude hero John Harrison is tested and declared a masterpiece

The best clocks now lose only a second every 300 million years – and those tiny time-keeping differences are changing our world.

Scientists working to create the perfect atomic clock have a fundamental problem: Right now, on the ceiling, time is passing just a bit faster than it is on the floor.

Russia will turn back its clocks for the last time on Sunday to permanently adopt winter hours. It will also increase its time zones from nine to 11, from the Pacific to the borders of the European Union. For the last three years, Russia experimented with keeping permanent summer time, but it proved to be highly unpopular with many Russians.

Physicists have verified a key prediction of Albert Einstein’s special theory of relativity with unprecedented accuracy. Experiments at a particle accelerator in Germany confirm that time moves slower for a moving clock than for a stationary one.

For eons, all manner of animals have lived their lives according to the cycles of the Earth’s rotation on its axis, the moon’s orbit around the Earth, and the Earth’s orbit around the sun. But why do we observe the week? The pattern of living on a seven-day cycle—with one or two of those days set aside for rest—is a relative novelty. Only in the past few centuries, with Western colonization of most of the world, have the majority of human societies adopted it.

The U.S. Department of Commerce has developed the most accurate atomic clock that will henceforth serve as the new U.S. civilian time and frequency standard.

A reformed calendar, with a pattern of two 30-day months, followed by one 31-day month, would be more business friendly

Device lays the groundwork for a new second.