Time is ubiquitous yet little understood. Scientists don't agree on many aspects of nature, and for the rest of us, there's either too little or too much of it. (Eugene Hoshiko/Associated Press)

Take a second to ponder this question.

What is time?

It's something that either passes too quickly, or with agonizing slowness.

It's the universal healer, an irresistible force that's not just part of our lives but the basis of them.

It is an omnipresent reality for everyone on the planet.

By definition, time is irreversible. Just ask the nearest middle-aged person, if you're not in that category yourself yet.

But we spend lots of hard-earned money trying to stave off the worst effects of aging.

We are constantly aware of time, yet it can still be nebulous and controversial by turns.

Just ask the Muslim scholars who recently urged Islamic countries to start orienting their clocks to the holy city of Mecca, rather than Greenwich in England.

Sir Sanford Fleming came up with the idea of standard time in 1876 when he missed a train in Ireland. (Canadian Press)

That East London suburb became the centre of the world for time thanks to Canada's 19th century engineer extraordinaire, Sir Sanford Fleming. He came up with the idea of standard time in 1876 when he missed a train in Ireland.

The burgeoning railway companies of 19th century Europe and the United States kept passengers guessing by observing their own times, based on the location of their headquarters or their main station. At one point in the late 1800s, Pennsylvania alone had seven different railroad time zones.

At first, Fleming, a Scottish-born Haligonian, wanted the whole world to agree on what time it was, but that proved impractical. So Fleming suggested instead that countries observe local times based on geography. The centre of the whole system would be Greenwich, where the Prime Meridian, zero degrees of longitude, passes through the ground of Britain's Royal Observatory.

All over the world, navigators carried reliable clocks called chronometers that were set to Greenwich Mean Time, GMT, They matched local time with GMT and the position of the sun to determine their longitude.

Back then, the French were early objectors. Paris tried but failed to come up with le temp francais mondiale. Eventually, France and anyone else who opposed the centrality of Greenwich settled for a scientific redefinition of GMT as Universal Coordinated Time, known as UTC and shorn of its pesky reference to perfidious Albion.

Proud of their time zone

Then there are the places that use time to assert national pride, often to the confusion or inconvenience of their citizens.

A digital clock in Beijing counts down the hours to the 2008 Summer Olympics. (Andy Wong/Associated Press)

China and India are huge countries with a single time zone centred on their capital cities. That means people in remote outposts have to live part of their daily lives in the dark, or sleep while it's light.

By contrast, Russia has 11 time zones and Canada six.

India has also offset its single time zone by half an hour, so it's not observing the same time as bitter rival Pakistan. Tiny Nepal goes one step further, displacing its local time a further 15 minutes from India, again so it's not in the shadow of a giant and sometimes overbearing neighbour.

Daylight savings time is also a sticky issue for some. Farmers don't like it, and places like Saskatchewan have avoided it for years. In Australia, there's even a tiny sliver of ranchland in the desert west of Sydney where locals refuse to turn their clocks ahead in defiance of national law.

Scientists believe time is a dimension. Albert Einstein showed that time and space are like length, breadth and height — part of a cosmic continuum that helps define our universe.

At the wilder extremes of theoretical physics, time can be a particle, a wave or even a fluid. Physicists refer to the "arrow of time" and wonder whether it can be changed at all, whether time travel is possible.

Einstein's theory of relativity demonstrated that time can slow down, especially if one is travelling at close to the speed of light — 1,079,252,849 km/h to be exact, a somewhat unlikely way to live a little longer.

Dr. Ronald Mallet, head of the physics department at the University of Connecticut has designed what he says could be a time machine, a powerful laser contained in a system of mirrors.

The beam of concentrated light grows in intensity, Mallet says, until it actually shifts time. He believes the device might even be used to send objects, even people, back in time; but only to the point where the machine was switched on.

Stephen Hawking, author of Brief History of Time, theorizes that time has a beginning, that before the universe appeared after the Big Bang, there was no time. What there was, Hawking doesn't say, an omission some of his peers find rather glaring.

Whatever its nature, there's no doubt that time is fodder for some of the world's greatest minds.

Overabundant and scarce at the same time

Pop culture, too, reflects our obsession with time.

Time may not be, as the Rolling Stones sang in 1964, on my side, and it's clear you can't save time in a bottle. However, time marches on and we live our lives as time goes by.

H.G. Wells had no difficulty inventing his time machine, in the pages of a novel released in 1912.

And the three Back to the Future films of the 1980s and early '90s certainly managed to tap our fascination with time, grossing more than $420 million U.S. for Universal Pictures.

At the end of the day, time will remain that rarest of commodities for us — something that can be both scarce and overabundant, even at the same time.

For that, we'll either be forever grateful or too busy to contemplate our good fortune.

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