Atomic clocks have changed how we calculate common units of time.
Back when humans began to track the passage of time thousands of years ago, they did it by watching the apparent movement of the sun across the sky -- which actually was due to the Earth's rotation -- and basing units of time on that journey. Traditionally, for example, a second was defined as 1/86,400 of the average length of a solar day.
But with the advent of atomic clocks, which were far more reliable than the motion of the Earth itself, it became necessary to change that standard. In 1967, the second was redefined as the time that it took for an atom of the isotope cesium 133 to oscillate 9,192,631,770 cycles [source: Sciencemuseum.org.uk].