Many dimensional theories include the assumption that powerful natural events provide anchor points, which lock together neighbours in a set of parallel universes. According to the strength of the uniting force, the links have a permanence in the analogy range from tacking stitches in an unfinished garment to spot-welds in the body of a motor car.
The eruption of the volcano on Krakatoa is invariably given as an example of a spot-weld which bound indissolubly the pasts of our local set of parallels. A more gentle tack was applied almost exactly eight months later when an earthquake shook the English county of Essex on the morning of April 22nd, Anno Domini 1884.
Even though natural events; passive eclipses and comets as well as violent upheavals; provide common reference points in parallel histories, agreement disperses as soon as they are fixed in terms of the local calendar. Different cultures on this Earth have solved in different ways, the problem of keeping track of the passage of time.
As most of them have chosen to number their years from a fairly arbitrary starting point, it is possible that the Roman origin could have survived the decay of the empire on another parallel Earth. Reformers with lunar inclinations could have added three months to a year out of phase with the seasons, instead of the two that were included in the Julian calendar.
In such a world, historians could record the date of Nature's stirring in Essex as May 6th, Ab Urbe Condita 2636. And equally, on a neighbouring parallel Earth on which slightly different views on the number of days to be assigned to the restructured months had prevailed, May 4th, AUC 2636.
The event remains common to all parallels: it took place when the Sun reached a specific point in the sky on a spring morning which arrived a fixed number of days after the vernal equinox. But human witnesses are noted for their divergent descriptions of the same event.
Herein lies the key to that which follows .....