The Doomsday Algorithm ( as mentioned here: https://devrant.com/rants/1000628 ) is the most harmless and wrongly named Algorithms I've ever come across. The basis of the Algorithm is the following:
an algorithm of determination of the day of the week for a given date. - Wikipedia (Doomsday Rule)
In short, you give it a date and it tells you the day of the week, as simple as that.
This was probably one of my first 'complex' programs I wrote back when I was probably somewhere between 12-14 years old, it was written in BASIC and honestly reading it now I can bearly make any sense of it myself.
The File is now available in it's original format 'Doomsday Algorithm.bbc' this WILL require the BBC BASIC editor to run the code, and in the future I will re write other language versions of this.