Calculate the day of the week for any date in your head

Re:Calculate the day of the week for any date in your head by kseistrup on Wednesday January 01, 2020 @03:53AM (#21927281)

To go from "any" century, you can also memorize "2053", which is the days in a 4-year cycle: 2000 = 2 = tuesday; 2100 = 0 = sunday; 2200 = 5 = friday; 2300 = 3 = wednesday.

For years other than the centuries, we need to add an offset. Given the year CCXX, we have:

    (a, b) = divmod(XX, 12)
    c = b // 4
    offset = a + b + c
    doomsday = start + offset

E.g., for 2020 (XX == 20) we get:

    start = 2  # from the "2053" rule
    (a, b) = divmod(20, 12)  # (1, 8)
    c = b // 4  # 2
    offset = 1 + 8 + 2 = 4 (mod 7)
    doomsday = start + offset  # 2 + 4 = 6 = saturday

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Re:Calculate the day of the week for any date in your head by ikeboy on Wednesday January 01, 2020 @08:55AM (#21928007)

The "add 11" trick in my comment is easier to remember and apply than dividing by 12 and taking remainders.

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Re:Calculate the day of the week for any date in your head by tzs on Wednesday January 01, 2020 @01:10PM (#21929440)

I like my current method [1], while only requires addition, doubling, and a couple of conditional increments or additions. Even if you don't bother doing any mod 7 reduction along the way, you never have to deal with a number above 32.

With both divide by 12 and odd 11, I would find myself occasionally making a goof when dealing with years near the end of centuries.

[1] https://news.ycombinator.com/item?id=21926543

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Re:Calculate the day of the week for any date in your head by kseistrup on Wednesday January 01, 2020 @09:09AM (#21928060)

I guess we have different perceptions of which way is the easiest. There might even be methods we haven't heard about yet.

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