I've been exploring mechanical and quartz designs a bit as of late, and stumbled upon this: Burgess Clock B Burgess Clock B, The World's Most Precise Pendulum Clock, Is Made To A 250-Year-Old Design By John Harrison, Longitude Prize Winner And Inventor Of The Marine Chronometer | Quill & Pad A mechanical pendulum clock that loses a bit over half a second over the course of 100 days. Then there is this World's Most Accurate Clock will Lose One Second Every 14 Billion Years | Digital Trends Which is an atomic clock that loses 1 second every 14 billion years. So the question for both is...how do they know? In the case of the mechanical clock, it was designed 250 years ago, and would have been the most precise time keeping measure at the time, so how could you know if it is losing half a second or not? Likewise with the 1 second in 14 billion years? Is it based on mathematics of what a "perfect" tick would look like and where the pendulum should be given a mathematically perfect tick? Is it based on calculations of where the sun/moon/stars should be, once again, given equations and seeing how that compares to what the respective clocks give?