Egyptians develop the first sundials (obelisks) to divide the day into 12 hours. Shadows cast by the sun indicated time.

Water clocks (clepsydra) invented in Egypt and Mesopotamia. Water dripped at a constant rate, measuring time regardless of weather.

Hourglasses appear in ancient Greece. Sand flowing through a narrow neck provided consistent time measurement for shorter intervals.

The Antikythera mechanism discovered - an ancient Greek analog computer that predicted astronomical positions and eclipses.

Su Song builds a massive astronomical clock tower in Kaifeng, China, using water-powered escapement mechanism.

Mechanical clocks with verge escapement appear in European cathedrals. These weight-driven clocks were inaccurate but revolutionary.

Pope Gregory XIII introduces the Gregorian calendar, replacing the Julian calendar to correct accumulated calendar drift.

Christiaan Huygens patents the first pendulum clock, achieving accuracy of seconds per day instead of hours.

John Harrison's H4 chronometer solves the longitude problem, enabling accurate navigation at sea within seconds per day.

Discovery that quartz crystals vibrate at precise frequencies when electrified, leading to modern quartz timekeeping.

Warren Marrison builds the first practical quartz clock at Bell Labs, accurate to 1 second per year.

National Institute of Standards and Technology (NIST) builds the first ammonia maser clock, predecessor to modern atomic clocks.

The International System of Units defines the second based on cesium-133 atom vibrations (9,192,631,770 cycles).

Optical lattice clocks achieve accuracy of 1 second in billions of years, the most precise measurements ever made.