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| The ] is for calculating the day of the week and makes use of the fact that in each year, | The ] is for calculating the day of the week and makes use of the fact that in each year, | ||
| 28/29 February ( |
28/29 February (28/2 or 29/2) | ||
| 4 April (4/4) | 4 April (4/4) | ||
| 6 June (6/6) | 6 June (6/6) | ||
| Line 8: | Line 8: | ||
| 12 December (12/12) | 12 December (12/12) | ||
| are all the same day of the week. This day of the week is called ] and is derived from the last day in February, be it the 28th or 29th. The algorithm also defines 9/5 and 7/11 and their reverses to be Doomsday. | are all the same day of the week. This day of the week is called ] and is derived from the last day in February, be it the 28th or 29th. The algorithm also defines 9/5 and 7/11 and their reverses (5/9 and 11/7) to be Doomsday. | ||
| Therefore, if you know what day of the week Doomsday - last day in February - is for a given year, you can easily determine the day of the week for any other date in that year. | Therefore, if you know what day of the week Doomsday - last day in February - is for a given year, you can easily determine the day of the week for any other date in that year. | ||
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| ==An example== | ==An example== | ||
| In the year 2004, |
In the year 2004, 29 February 2004 is Sunday | ||
| Hence Doomsday for 2004 is Sunday | Hence Doomsday for 2004 is Sunday | ||
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| 7 March (7/3) | 7 March (7/3) | ||
| As an example to find the day of the week for |
As an example to find the day of the week for 17 April: | ||
| We know April |
We know 4 April (4/4) is a Sunday, | ||
| So is |
So is (4+7) April, | ||
| And |
And (4+14) April too, | ||
| That is |
That is 18 April is a Sunday, and hence | ||
| -April |
-17 April 17 is a Saturday- | ||
| ==External links== | ==External links== | ||
Revision as of 07:26, 9 November 2004
The Doomsday algorithm is for calculating the day of the week and makes use of the fact that in each year,
28/29 February (28/2 or 29/2) 4 April (4/4) 6 June (6/6) 8 August (8/8) 10 October (10/10) and 12 December (12/12)
are all the same day of the week. This day of the week is called Doomsday and is derived from the last day in February, be it the 28th or 29th. The algorithm also defines 9/5 and 7/11 and their reverses (5/9 and 11/7) to be Doomsday.
Therefore, if you know what day of the week Doomsday - last day in February - is for a given year, you can easily determine the day of the week for any other date in that year.
The Doomsday algorithm was invented by John Horton Conway.
An example
In the year 2004, 29 February 2004 is Sunday Hence Doomsday for 2004 is Sunday
Hence, we deduce that the following days in even months are sundays
4 April (4/4) 6 June (6/6) 8 August (8/8) 10 October (10/10) and 12 December (12/12)
For odd months remember 9/5 7/11. That means that the following are sundays too:
9 May (9/5) 11 July (11/7) 5 September (5/9) 7 November (7/11)
For January and March, its quite different However. The Doomsdays are:
3 January (3/1) - for non leap years 4 January (4/1) - for leap years
and for March the doomsday is
7 March (7/3)
As an example to find the day of the week for 17 April:
We know 4 April (4/4) is a Sunday, So is (4+7) April, And (4+14) April too, That is 18 April is a Sunday, and hence -17 April 17 is a Saturday-
External links
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