Einstein's theory of relativity says that time passes more slowly the closer you get to the ground, because you're closer to Earth's gravitational field. It's a tiny difference, but big enough for a pair of laser-powered clocks to actually measure.
Time dilation is one of the more famous aspects of Einstein's theory, although most people are only familiar with the special relativity version of this phenomenon. In that case, time moves at different speeds depending on how fast you're moving relative to the speed of light.
The classic example is of two observers, one on a speeding train and the other standing on the station platform. Time is moving ever so slightly more slowly for the observer on the train than the observer on the platform. It's an almost imperceptible difference, but it's real - in fact, some astronauts are a few seconds younger than they would have been if they had never left Earth. And time actually doesn't pass at all if you're traveling at the speed of light, which means any sentient photons out there haven't perceived any time elapse since the moment of the Big Bang.
So that's the special relativity version of time dilation. The general relativity version is a byproduct of the theory's basic idea, which states gravity is an effect caused by objects warping the fabric of spacetime. More massive objects, like the Earth or the Sun, obviously create a much deeper indentation in spacetime, but everything and everyone is bending spacetime a little to create their own little gravitational field.
But as long as you're on this planet, Earth is the overwhelming gravitational force, and the stronger the gravity, the slower the passage of time. Maybe the most famous example of this is with black holes, as any object approaching the event horizon will appear to slow down to a standstill due to the tremendous gravitational forces. But even on Earth, that effect exists, and the difference is about 3 microseconds per kilometer per year. According to my calculations, that means the flags Edmund Hillary and Tenzing Norgay left on the summit of Mount Everest in 1953 are about .0015 seconds older than if they had stayed at sea level.
That's not much of a difference, of course, and it's not like they left a clock on top of Everest to actually demonstrate this effect. However, atomic clocks on jets have been used to prove this effect over a difference of several kilometers, and now researchers at the National Institute of Standard and Technology in Boulder, Colorado, have managed to show a discernible difference between the passage of time on ground level and a few feet above ground level - in other words, on a human scale.
Now atomic clocks, which uses the constant frequencies of atoms jumping between energy states to keep time, are generally thought of as the world's most accurate timekeepers. And for most official purposes, atomic clocks are the preferred option - for instance, the international time standard is set atomic clocks with a frequency of 9.2 billion cycles per second. But that isn't nearly precise enough to measure such a subtle relativistic effect.
To do that, the NIST researchers had to use optical clocks that cause an aluminum ion to switch energy states even more quickly, changing states over a quadrillion times every second. The actual gain in accuracy isn't quite that dramatic - it's only about forty times more accurate than the best atomic clocks, but that's enough to spot the almost infinitesimal effect relativity has on a human scale.
The researchers were able to show a clear time difference between two optical clocks separated by only fifty centimeters. They were also able to show a special relativity time dilation effect at speeds as slow as four meters per second - that's less than half the speed at which current world's fastest man Usain Bolt runs the 100 meter dash, although admittedly he might be a bit slower if he had to carry a heavy optical clock.
Either way, it looks like runners really are keeping themselves younger, even if it's only by a couple billionths of a second. And if you want time to just fly by...well, invest in mountaintop properties. Or just try and grow a couple inches taller.
[Science; pictured up top is the world's very first atomic clock.]