Atomic clocks are incredibly reliable now, but the first ones weren't.
In 1948, the U.S. National Bureau of Standards built the world's first atomic clock. Instead of cesium, the first clock used ammonia atoms, which were heated and shot out of a copper pipe. While the first clock proved that the concept of atomic clocks worked, it was never actually used for time keeping. The first atomic clock was off by about one second every four months. That made it less reliable than an existing technology, the quartz clock, which measured the oscillation of a piece of quartz when an electrical charge was applied to it.
Eventually, the scientists switched to using cesium, which had shorter oscillations, and improved the design in various ways. A 1959 model managed to keep time with an error of one second per 2,000 years, and by 1964, the clocks had become so precise that it took 6,000 years for them to lose or gain a second. Today a state-of-the-art atomic clock would be off by just one second after 6 million years of use [source: Sciencemuseum.org.uk].