For me (not GP), this lives in the recreational-mathematics part of my brain rather than the useful-things part of my brain: John Conway called this invention o

For me (not GP), this lives in the recreational-mathematics part of my brain rather than the useful-things part of my brain: John Conway called this invention of his the “Doomsday rule”[1] and I suppose the memorable name helps. But I admit having opened Calendrical Calculations[2] once or twice as well, so perhaps my idea of fun is skewed. [1] https://en.wikipedia.org/wiki/Doomsday_rule https://en.wikipedia.org/wiki/Doomsday_rule [2] https://en.wikipedia.org/wiki/Calendrical_Calculations https://en.wikipedia.org/wiki/Calendrical_Calculations