[boost] date_time: ISO week number error and Calendar FAQ references

From: Claus Tondering (claus.tondering_at_[hidden])
Date: 2003-09-25 05:23:58

The Boost date_time source code kindly refers to my Calendar FAQ
document.

I have a few comments in this connection:

1) In boost/date_time/gregorian_calendar.ipp I would appreciate it if
   you would spell my name correctly (it's Tondering, not Todering).

2) In the same file you use an algorithm for calculating the ISO week
   number found in the April 2000 version of the Calendar FAQ. You
   have, however, made an error when translating the algorithm from
   English into C++:

   In the function ...::week_number(const ymd_type& ymd), this code:

      else if (day != 5) {
        return 1; //monday - thursday is in week 1 of next year
      }

   should be replaced with this code:

      else {
        return 1; //monday - wednesday is in week 1 of next year
      }

   (The current Boost algorithm returns a week number of 53 for
   2003-12-29. But the correct value is 1.)

3) Incidentally, newer versions of the Calendar FAQ contain a better
   algorithm for calculating the week number:

      d4 = (J+31741 - (J % 7)) % 146097 % 36524 % 1461;
      L  = d4/1460;
      d1 = ((d4-L) % 365) + L;
      WeekNumber = d1/7+1;

   where J is the Julian Day Number. (All variables are integers.)

4) In libs/date_time/doc/References.html you should use this URL for
   the Calendar FAQ:

        http://www.tondering.dk/claus/calendar.html

--
Claus Tondering, Lyngby, Denmark
Email: claus_at_[hidden]