It's also a beautiful maze of nerd snipes. There's apparently some shifting going on inside the earth, combining with the slight variability of the moon's distance, which means various days over a month can be more than a millisecond shorter than average. Good luck integrating that into your date.addDays()

The more precisely you try to measure anything the more confounding factors you encounter. Take length as an example: every material has some coefficient of thermal expansion, and that coefficient can itself change with temperature. Even Zerodur. Every material has a finite speed of sound. If you measure length precisely enough and fast enough, you'll find every material changes length as ambient sounds pass through. So everything is a thermometer, and a microphone, and more since there are many other "parasitic" variables.