The need to tell the time connects you to over five thousand years of history, from the first solstice markers to the quartz crystal oscillating in your watch, and from orbital motion to our ultra-precise atomic clocks. Yet time is also a social construct: the Gregorian calendar is a result of complex negotiations. Learn about the wondrous physics that makes time something we can set and measure. The science of marking time, from Stonehenge to atomic clocks.