A Brief Economic History of Time
Long before the modern clock used springs and familiar markings, just about every great civilization had attempted to measure time, with each one failing in its own special way. In ancient Egypt, China, and Mesopotamia, sundials, or “shadow clocks,” all required bright sunlight to count the hours, which wasn’t of much use on overcast days. To work around this problem, some of these ancient civilizations used a “water clock,” or clepsydra, a device that steadily dripped water through a small hole into a container with lines painted around the side to represent the passage of time. But slight changes in temperature could change the viscosity of water and the rate of drips. On a cold day, the water might freeze, and so would time… More at The Atlantic.
Quantum Gravity’s Time Problem
In quantum mechanics, time is universal and absolute; its steady ticks dictate the evolving entanglements between particles. But in general relativity (Albert Einstein’s theory of gravity), time is relative and dynamical, a dimension that’s inextricably interwoven with directions x, y and z into a four-dimensional “space-time” fabric. The fabric warps under the weight of matter, causing nearby stuff to fall toward it (this is gravity), and slowing the passage of time relative to clocks far away. Or hop in a rocket and use fuel rather than gravity to accelerate through space, and time dilates; you age less than someone who stayed at home… More at Quanta Magazine.
Some deep reading to get you to 2017, especially the second article.