Doomsday Calculator

The Doomsday Algorithm is a method invented by mathematician John Conway for mentally calculating the day of the week for any date. It works by finding a set of anchor dates (called 'doomsdays') that always fall on the same weekday within a given year, then counting forward or backward from the nearest doomsday to your target date.

In the Doomsday Algorithm, a 'doomsday' refers to a specific set of memorable dates within any given year that all fall on the same day of the week. Examples include 4/4, 6/6, 8/8, 10/10, 12/12, the last day of February, and 5/9, 9/5, 7/11, 11/7. Knowing which weekday these fall on in a given year lets you quickly work out any other date.

The anchor day depends on the century of the year. For 1800–1899 it is Friday, for 1900–1999 it is Wednesday, for 2000–2099 it is Tuesday, and for 2100–2199 it is Sunday. These anchor days repeat in a 400-year cycle, shifting by two days with each new century.

Dividing the last two digits of the year by 12 (integer division) is the first step of Conway's algorithm for finding the year's doomsday. The quotient, remainder, and how many times 4 fits into that remainder are all added together with the century anchor to produce the year's doomsday number, which maps to a day of the week.

Yes. The Doomsday Algorithm and this calculator work for any year under the Gregorian calendar system, including dates in the 1800s, 1700s, and further back, as well as future dates. The century anchor day automatically adjusts based on the century you enter.

This calculator uses the Gregorian calendar throughout. Many countries switched from the Julian calendar at different points in history, so for dates before a country's adoption of the Gregorian calendar, the calculated weekday may differ from contemporary historical records, which used the Julian calendar.

Yes — with practice! John Conway designed the algorithm to be performed mentally. The key steps are: find the century anchor, compute the year adjustment using 12s and 4s, identify the nearest doomsday date to your target, and count the difference. Many people memorize the anchor days and doomsday dates to calculate any weekday in seconds.

Leap years are accounted for in the doomsday dates for January and February. In a leap year, January 4th and February 29th are doomsday dates; in a non-leap year, January 3rd and February 28th are used. The calculator handles this automatically when computing the final day of the week.