Except for the fact that there was certainly no month more that 10^20 (or so) years ago, because the universe didn't exist yet and/or more specifically, time as we define it doesn't reach that far into the past. But yes, the answer to the original question relies only on residues (mod 7) of powers of 10 , so it is essentially the same for any calender that has weeks of seven days and no exceptions to that. It is quite obvious that one particular value will never appear as such a residue, slightly less obvious that all others will (and do so for N < 7 — incidentally corresponding to roughly the age of mankind). - Maximilian On Tue, 19 Jan 2021, 22:56 Keith F. Lynch, <kfl@keithlynch.net> wrote:
I have done exactly that, and confirmed that *every* day can indeed be expressed as day 10^N of some prior month in infinitely many ways, and that the most recent such N never exceeds 306.
(...)
September 10^4, 1993, it was also September 10^1162 of an enormously earlier year, September 10^2320 of an even earlier year, ad infinitum. Phrased another way, 10^1159 = 1 (mod 146097).