Already does that-- `sdate --covid 19` will keep it perpetually March 2020. It also accepts arbitrary starting points with `--epoch`.

Not sure how you'd do a perpetual 2020 while allowing the month to advance-- pretty simple if you keep the month as a number, I suppose, but problematic for the named months used in the examples (what should the 44th month of a year be named?).

You mean like this XKCD? https://xkcd.com/2459/