On 25th of December 1656 the Dutch astronomer and mathematician Christiaan Huygens designed the first pendulum clock.

Drawing of the first pendulum clock, designed by Dutch scientist Christiaan Huygens in 1657. Huygens contracted his clock designs to clockmaker Salomon Coster of The Hague, who actually built the clock. The pendulum had a wider swing than modern clocks, perhaps 80°-100°, due to its verge escapement. The drive force for the clock was provided by the two weights at bottom in an ingenious “endless rope” mechanism.

It was patented the following year.

Huygens contracted the construction of his clock designs to clockmaker Salomon Coster, who actually built the clock. Huygens was inspired by investigations of pendulums by Galileo Galilei beginning around 1602. Galileo discovered the key property that makes pendulums useful timekeepers: isochronism, which means that the period of swing of a pendulum is approximately the same for different sized swings. Galileo in 1637 described to his son a mechanism which could keep a pendulum swinging, which has been called the first pendulum clock design. It was partly constructed by his son in 1649, but neither lived to finish it. The introduction of the pendulum, the first harmonic oscillator used in timekeeping, increased the accuracy of clocks enormously, from about 15 minutes per day to 15 seconds per day leading to their rapid spread as existing ‘verge and foliot‘ clocks were retrofitted with pendulums.

Wikipedia