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Help on the doomsday equation/birthday equation

  1. Jan 28, 2007 #1
    here's the equation

    d = date of the month
    m = number of the month in the year
    y = year

    W = d + 2m + [3(m+1)/5] + y + [y/4] - [y/100] + [y/400] + 2

    afterwards, divide w by 7, and the remainder is the day of the week, according to:

    1 - sunday
    2 - monday
    3 - tuesday
    4 - wednesday
    5 - thursday
    6 - friday
    0 - saturday

    now, i totally understand the equation, and it works. but i just dont understand what the significance of the [y/4] [y/100] and [y/400] is.
    why is it needed in the equation? :frown:
  2. jcsd
  3. Jan 28, 2007 #2


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    Dearly Missed

    Take them out. Does the formula work then?
  4. Jan 28, 2007 #3
    no, the formula is off. when i tried it with today's date without them, it turned out 1 day behind.
  5. Jan 29, 2007 #4


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    Think about leap years.
  6. Jan 29, 2007 #5
    I tried this equation for Monday, January 29 and it did not work. It also did not work for Tuesday, January 30 or Monday, January 01. It always comes up one day behind. Does it stop working at some year?
  7. Jan 30, 2007 #6


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    Think about leap years!
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