>>52863071Because Glimmora is possibly connected to evolution, it could mean that those crystals are the best material reference point in highlighting certain "eras" in the pokemon world. So it makes it easier to estimate when the time machine could go back. Then the same process could be used to extrapolate forward coming eras too.
Basically using plant/tree rings and terrestrial layers to denote geological time related events. But this is all head canon.