### Doomsday Rule Algorithm

March 11, 2020 in Software Engineering | 1 min read | Tagged: javascript user-interface codepenAn algorithm to know the day of the week for a given date (method valid for dates from 1800s to 2000s).

How can you quickly and accurately calculate the day of the week associated with a specific date?

Here is a link to a working demo utilised using JavaScript: Check it at this Code Pen page

I explained the instructions on the demo with working example, but here are the method to do that:

- Start by taking the last two digits of the year, this is your startingNumber.
- Divide the startingNumber by four and ignore the remainder.
- Add the result to startingNumber.
- Find the month code: January: 1, February: 4, March: 4, April: 0, May: 2, June: 5, July: 0, August: 3, September: 6, October: 1, November: 4, December: 6,
- In leap years, subtract one from the month code for January and February.
- Find if the year is leap. (Century years must divisible by 400, non-century year, last two digits must divisible by 4)
- Add the month code number.
- Add the day of the month.
- If year is in 2000s subtract 1, if year is in 1800s add 2.
- Find the remainder of dividing the total by 7.
- Find the day of the week according to the day codes. Starting Saturday: 0, Sunday: 1, … , Friday: 6

Give it a try at Code Pen if you need an example to learn something.