1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    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


    User Avatar
    Science Advisor

    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


    User Avatar
    Science Advisor

    Think about leap years!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook