Doomsday Algorithm

I’m not entirely sure why it took me so long to find a system, like this, that works in my head! Or, your hands:

Dr. Conway now teaches the Doomsday algorithm, complete with Century adjustment, using a very simple visual aid—your hand.

      _____
 ____/  ___)____   <-- 1
         _______)  <-- 2
         ________) <-- 3
 ____    _______)  <-- 4
     \________)    <-- 5

1 — Doomsday Difference
2 — Century Day
3 — number of DOZENS
4 — remainder
5 — number of 4s in remainder

The Doomsday Difference is the difference between the required date and a nearby Doomsday, recorded as so many days “on” (i.e. to be added) or “off” (subtracted) from that Doomsday.

So cool.