Clocks with quartz movements are more accurate at keeping time than those that use pendulums. Quartz has largely replaced pendulums in modern clocks. But, in their day, pendulum clocks were profoundly important due to their ability to measure time accurately over long periods. The first pendulum clocks were produced in the mid 17th century and their impact was dramatic. Their use ushered in a new era of accurate time keeping. The reliability of pendulum clocks is based on the predictable relationship between the length of a pendulum and the time it takes the pendulum to complete one full swing.