Learn the Algorithm
Step 1 — Century Anchor
Each century has a fixed “anchor day” — the day of the week that Doomsday falls on for years in that century.
| Century | Anchor Day | Value |
|---|---|---|
| 1800s | Friday | 5 |
| 1900s | Wednesday | 3 |
| 2000s | Tuesday | 2 |
| 2100s | Sunday | 0 |
General formula:
(5 × (century % 4) + 2) % 7Worked Example — Sep 11, 2001
Century:2000s → Tuesday (2)
Try Any Date
Pick a date and see the full Doomsday calculation in real time.
September 11, 2001 → Tuesday
01Century Anchor
Century:2000s → Tuesday (2)
02Year Doomsday
Y =01
a =01 ÷ 12 = 0
b =01 % 12 = 1
c =1 ÷ 4 = 0
Doomsday =(2 + 0 + 1 + 0) % 7 = 3 → Wednesday
03Month Anchor
September anchor date:September 5
September 5, 2001 =Doomsday (Wednesday)
04Count to Target
Target:September 11
Offset:11 − 5 = +6
Wednesday + 6 =Tuesday ✓