First Sunday Doomsday Algorithm

Here's a full-year perpetual calendar cup. Just one moving part, and you adjust it just once a year. All the months end properly at 30 o...

Here is a 200-year calendar in the form of a handy pencil cup.  I made it from PVC pipe, with laser-printed "sticker paper", and l...

I set myself a goal: to make the simplest device that can find the day of the week for any date. I think you'll agree that a device made...

[Revised 7/29/2011] This quick summary will show you, by example, how to determine the day of the week for any date -- in your head! (See...

[Revised 1/23/2023] This longer article will show you how to determine the day of the week for any given date -- without consulting a cale...