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Doomsday algorithm
Given a date, determine the day of the week it falls on. All based on last day of February in the year, the doomsday. Several places have info from which I've put this together for the Lexington Computer Group; see Wikipedia's entry.
calendars for demo
Century
| Weds | Tues | Sun | Fri |
| 1900 | 2000 | 2100 | 2200 |
| 1500 | 1600 | 1700 | 1800 |
- 1700: Sun shined on USA SUNDAY
- 1800: Ate = Fries = FRIDAY
- 1900: We'd this century WEDNESDAY
- 2000: Tue-thousand TUESDAY
Computing
This is the odd-plus-eleven method.
- Determine anchor day of Century.
- Now work with the last 2 digits of year.
- Is year odd? If so add 11
- Divide by 2
- Is result odd? If so add 11
- Divide by 2
- Take mod 7
- Subtract this from 7; add resulting number of days to anchor day for the year's doomsday
Now compute day given month.
| Jan | Jan 3 unless it's leap year then Jan 4 |
| Feb | Feb 7 unless it's leap year then Feb 8 |
| Mar | Mar 7 |
| apr, june, august, october, december | month number (October is 10th month so Oct 10) |
| 9-to-5 and 7-11 | Sept 5, May 9th, July 11, Nov 7 |
Examples
Signing declaration of independence, July 4, 1776.
1700: anchor is Sunday
76 is even, /2 is 38, is even, mod 7 is 3, from 7 is 4, Sunday + 4 is Thursday is doomsday for 1776.
July 4, 7th month 11th day, hence 4th day, falls on Thursday.
Day of moon landing, July 20, 1969
1900: anchor is Wednesday
69 is odd +11 is 80 /2 is 40 is even, mod 7 is 5, from 7 is 2, so doomsday for 1969 is Friday
July 11 and 18 are doomsday, Friday, so July 20 is Sunday.
Signing WW1 armistice, Nov 11, 1918.
1900: anchor is Wednesday
18 is even, /2 is 9, 9 is odd +11 is 20. Mod 7 is 6, from 7 is 1, so doomsday for 1918 is Thursday
Nov 7 is Thursday, so 11th is Monday.