Ah, but for three years ago or five years hence? I can’t even begin to guess those numbers…

I guess I’d have to devise some way to figure it out on paper. Back in the early 2000s when I wrote my calendar sidepanel one of my primary resources for devising formulas for things like Easter was an earlier version of this site: http://www.tondering.dk/claus/cal/christian.php

I must say I kind of miss when it was one gigantic page with just black on white text.

Anyway, my answer to your conundrum would be this:

$ date --date="21 June 2002"
Fri Jun 21 00:00:00 CEST 2002

I suppose the question is whether I could come up with that by hand… oh well, let’s hope I don’t run out of electricity during my lifetime. ;)

plentiful calendars within easy reach

Ah, but for three years ago or five years hence? I can’t even begin to guess those numbers…

much easier to count it out in my head or on my knuckles

Mary always impresses me by doing date calculations in her head: how many days between X and Y, Z will fall on which day of the week, stuff like that. I’m reduced to fingers and fumbles…

Dertig dagen heeft November,
April, Juni en September.
De andere hebben dertig en één,
Uitgenomen Februari alleen,
Want die heeft er viermaal zeven;
‘t Schrikkeljaar nog één daarneven.

Thirty days has November,
April, June, and September. (singular verb is not my mistake)
The others have thirty-and-one,
except February alone,
because it has four times seven;
the leap year still one in addition.