¡SkyCaramba! Weekly astronomy blog for the week ending November 23, 2013
For thousands of years, telling the time was inextricably related to noting the positions of objects in the sky. When the sun was highest in the sky, people agreed it was noon. Some people needed more accurate
scheduling than saying before noon, after noon, or night. They developed various methods of dividing the day into smaller pieces. Thousands of years ago, candle clocks marked time by how much wax had burned away. Water clocks did so by how much water dripped in or out of a container.
In the 1200s, as Europeans started thinking of ways to make machines to assist them in their work, a few engineering people developed clocks that worked by directing and redirecting energy stored in a pendulum. As the pendulum swung back and forth, it turned gears, pulled on bars and rods, and ultimately moved a pointer from which a person could read the time.
Moving a pendulum clock required taking it out of service. So if you wanted to know what time it was, you went to the town hall or the church where the clock sat. Eventually, someone figured out how to use a spring to replace the pendulum and that’s when mechanical clocks became portable. Because a spring clock ticks 
whatever position it’s in, even sailors could tell time with one on a ship rocking on the high sea. Millions of people eventually carried mechanical clocks so miniaturized, they fit in their pockets or could be strapped to their wrists.
In the 20th Century, scientists found that a steady electric current going through certain kinds of crystals produced a sort of on-off switching activity. The switching happened so regularly and the materials were so inexpensive, battery operated quartz timekeeping mechanisms became an alternative to wind-up wristwatches. At an even smaller level, scientists learned that atoms vibrate regularly enough to be used as clock mechanisms.
As timepieces became more accurate, timekeepers became more aware of how inaccurate previous timepieces had been. By the 1950s, scientists had to confront the fact that the original timepiece—Earth itself—isn’t as accurate as we all like to think. Astronomers had known for hundreds of years about seasonal variations that made the solar day a little longer or a little shorter than 24 hours depending on the time of year. But underneath that, the atomic clock revealed a non-seasonal variation in Earth’s rotation.
Today, scientists in charge of atomic clocks keep them set to Terrestrial Dynamic Time (TDT). It’s currently about 67 seconds ahead of Coordinated Universal Time (UTC). UTC is what governments, broadcasters, and most businesses use with an adjustment to their local time zones.
UTC is kept close to the time that would be reckoned by the solar day. That’s done by inserting a leap second at the end of June or December, but not every June and December. The last time it happened was in June 2012. Before that, the end of December 2008. The need has decreased since the 1970s, 1980s, and 1990s, when a leap second was inserted nearly every year. Whether the need will continue decreasing in the future, we can’t know.
The good news for you is you don’t have to worry about it. Even if you have a so-called atomic clock hanging on your wall, the radio signal it tunes to carries a time stamp already adjusted for UTC.
¡SkyCaramba!
http://www.staff.science.uu.nl/~gent0113/deltat/deltat.htm
http://listverse.com/2013/06/26/10-ingenious-ancient-time-keeping-devices/