Comment by lefra

By adding up the uncertainties of all intermediate measurements they made.

As a parallel, for a regular clock, you'd measure and compute the uncertainties of:

- The distance between the axis of rotation and the center of mass of the pendulum,

- The local acceleration of gravity.

And deduce the pendulum period and its uncertainty by calculus.

In that case I'm not sure you can build (and measure) a pendulum that's more precise than a previous realization of the second (because the meter is defined from the speed of light and the second, and gravimeters probably depend on the definition of the second too).

That's why the definition of base units is carefully chosen to not depend on previous realization. IIRC time is the most fundamental of base units, because other base units depend on it.