arXiv:1010.0765cs.DS
Methods for Accelerating Conway's Doomsday Algorithm (part 2)
Abstract
We propose a modification of a key component in the Doomsday Algorithm for calculating the day of the week of any calendar date. In particular, we propose to replace the calculation of the required term: \lfloor \frac{x}{12} \rfloor + x \bmod 12 + \lfloor \frac{x \bmod 12}{4} \rfloor with -[ \frac{x+11(x \bmod 2)}{2} + 11 (\frac{x+11(x \bmod 2)}{2}\bmod 2)] \bmod 7 for a 2-digit input year x; Although our expression looks daunting and complicated, we will explain why it is actually easy to calculate mentally.
For figuring the doomsday of a particular year, you can use the Odd+11 rule. Let X be the last two digits of a year. If X is odd, add 11. Divide X by 2. If X is odd, add 11. Let Y = 7 − (X mod 7). Count forward Y days from that century's anchor day to get the year's doomsday.…
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