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The Gregorian Calendar

Posted By: ah_clem
Date: Saturday, 14 May 2011, at 1:52 a.m.

In Response To: The Gregorian Calendar (David Rockwell)

As others have been saying cryptically, it's about the cycle. the Gregorian calendar is 400 years long and contains 365*400 + 97 days (leap years) That number is 146097 and it is divisible by seven. So, not only do the dates repeat, the days of the week do too, and since the cycles repeat endlessly, if 1/7th of the 13ths do not fall on Fridays in a single 400 year cycle, extending the time period won't change anything.

So, it's just a matter of counting how many of the 146097 days are the 13th of the month and observing that this number is not divisible by 7. Left as an exercise for the reader.

Messages In This Thread

• Friday the 13th
  Bob Koca -- Friday, 13 May 2011, at 7:22 p.m.
  • Friday the 13th
    Stein Kulseth -- Friday, 13 May 2011, at 9:39 p.m.
  • Friday the 13th
    Rich Munitz -- Friday, 13 May 2011, at 10:35 p.m.
    • Friday the 13th
      Casper van der Tak -- Friday, 13 May 2011, at 10:49 p.m.
      • Friday the 13th
        Bob Koca -- Friday, 13 May 2011, at 10:50 p.m.
        • Friday the 13th
          Casper van der Tak -- Friday, 13 May 2011, at 10:55 p.m.
          • Friday the 13th
            Stein Kulseth -- Friday, 13 May 2011, at 10:59 p.m.
            • Friday the 13th
              Casper van der Tak -- Friday, 13 May 2011, at 11:04 p.m.
            • The Gregorian Calendar
              David Rockwell -- Saturday, 14 May 2011, at 1:12 a.m.
              • The Gregorian Calendar
                ah_clem -- Saturday, 14 May 2011, at 1:52 a.m.
                • The Gregorian Calendar
                  David Rockwell -- Saturday, 14 May 2011, at 2:26 a.m.
                  • The Gregorian Calendar
                    Timothy Chow -- Saturday, 14 May 2011, at 2:47 a.m.
                    • The Gregorian Calendar
                      David Rockwell -- Saturday, 14 May 2011, at 11:11 p.m.
                      • The Gregorian Calendar
                        Sam Pottle -- Sunday, 15 May 2011, at 1:10 p.m.
                      • The Gregorian Calendar
                        Elliott Winslow -- Wednesday, 18 May 2011, at 4:02 a.m.
                • The Gregorian Calendar
                  Casper van der Tak -- Saturday, 14 May 2011, at 4:53 a.m.
                • The Gregorian Calendar
                  Rich Munitz -- Saturday, 14 May 2011, at 3:00 p.m.
    • Friday the 13th
      Stein Kulseth -- Friday, 13 May 2011, at 10:58 p.m.
  • Friday the 13th
    Nack Ballard -- Saturday, 14 May 2011, at 1:57 p.m.
    • Friday the 13th
      Casper van der Tak -- Saturday, 14 May 2011, at 2:23 p.m.
      • Friday the 13th
        Nack Ballard -- Tuesday, 17 May 2011, at 4:19 p.m.
        • Friday the 13th
          Casper van der Tak -- Tuesday, 17 May 2011, at 5:13 p.m.
      • Friday the 13th
        ah_clem -- Monday, 23 May 2011, at 4:04 p.m.
        • Friday the 13th
          Casper van der Tak -- Monday, 23 May 2011, at 4:32 p.m.
          • Friday the 13th
            ah_clem -- Tuesday, 24 May 2011, at 1:47 a.m.
            • Friday the 13th
              Casper van der Tak -- Tuesday, 24 May 2011, at 7:30 a.m.
  • Friday the 13th
    garyo -- Saturday, 14 May 2011, at 4:58 p.m.