Most of us here in the USA set our clocks back one hour on Saturday night (back to Standard Time). A state such as Arizona is on Mountain Standard Time all year. And these days only dumb, disconnected mechanical clocks require adjustment. My iPhone’s little clock is smart and changes on its own.
So where does my iPhone’s time come from? Well, it comes from GPS satellites that broadcast the time to cell towers, from where the smartphones pick it up. And GPS satellites get their times from atomic clocks at the US Naval Observatory (a whole bunch of them).
In 1967, scientists got together and defined one second as equivalent to the time it takes a cesium atom to move 9,192,631,770 times between two particular energy levels. That defines the time in terms of 16 decimal points of a second. Is that really necessary? Well, yes .. GPS satellites that are out of sync by as little of one billionth of a second with the master time will already result in inaccuracies of a few feet on the ground, so these GPS satellite times are literally synced to the nanosecond. Researchers are working on atomic fountain clocks, to push the definition of time out to 18 decimal points (a hundred times more accurate than today’s clocks, or an accuracy of 1 second in 300 million years). But at that point, relativity and quantum effects start to take hold : an atomic clock a few feet higher than one right next to it will consistently run slower than one at the lower height due to the differences in the earth’s gravity at the two points.
For further explanations and speculations on time measurement, check out this little video clip from Wired Magazine/ The Atlantic magazine link.