Actually friction changes the frequency, see [1]. Moreover there is this general idea in spectroscopy (whether of atoms or of pendulum clocks) that to precisely

Actually friction changes the frequency, see [1]. Moreover there is this general idea in spectroscopy (whether of atoms or of pendulum clocks) that to precisely measure a frequency you need the smallest dissipation possible (check wikipedia about linewidth-lifetime relations, Q-factors, etc). Basically the precision with which you can measure a frequency is 1/lifetime_of_the_phenomenon. Higher dissipation means lower precision. See [1] https://en.wikipedia.org/wiki/Harmonic_oscillator#Damped_harmonic_oscillator https://en.wikipedia.org/wiki/Harmonic_oscillator#Damped_har... edit: If this change of frequency is calibrated it is not an issue, but this calibration is not trivial and friction is something a lot less stable than the other physical properties that govern the clock.