01 · DATUM — the reference frame

The shift is cheap.
The baseline is the instrument.

A restated number is parallax: the same fact, seen from two viewpoints, lands in two places. Astronomy turned that into measurement by citing the separation between viewpoints. Fundamentals data mostly doesn't — it overwrites, and the shift is lost. Here the viewpoints are pinned (as-of day D vs. today), the shift is measured, and the proof is written down where you can check it.