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Newsgroups: comp.databases,comp.software-eng
Subject: Re: Calendar Algorithm
Keywords: calendar algorithm
Message-ID: <1145@kluge.fiu.edu>
Date: 10 Aug 90 21:40:36 GMT
References: <140352@sun.Eng.Sun.COM>
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Followup-To: comp.databases
Organization: Florida International University, Miami
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In article <140352@sun.Eng.Sun.COM> jasonf@cetemp.Eng.Sun.COM (Jason Freund) writes:
>
>	I need to be able to change dates in a database cell with commands like
>"+4w" == "Find the closest weekday to the day 4 weeks from a certain date", 
>"-12d" == "Find the closest weekday 12 days earlier than this date.", and so 
>on.  I'll be writing in Pascal -- the code is no problem -- I'll give my 
>function a "+/-", a number, and a "d/w/m" (day, week, month), and a date 
>"MMDDYY".  The function returns the closes weekday by adding/subtracting 
>according to its parameters.  Does anyone know of a general algorithm for 
>dealing with calendars?  It must that take into account leap years and be able 
>to find what day of the week it is (MTWRF) so that it can make approximations 
>to weekdays. I've seen some good ones before, (for computing how many days old 
>you are and what day of the week you were born on) but I can't find any now.  
>


   function canonical_day_number( date: YMD_date ) return integer is
      -- Expects a year, month, day combination and returns an integer
      -- which is the number of days since a very long time ago.  To
      -- find the number of days between two dates, subtract these numbers.
      -- To find the day of the week, take the result mod 7.
      year2  : integer := date.year;
      month2 : integer := date.month;
   begin
      if month2 < 3 then 
         month2 := month2 + 12; 
         year2 := year2 - 1; 
      end if;
      -- The following code avoids using floating point or really big numbers.
      return year2 * 365 + year2 / 4 - year2 / 100 + year2 / 400
            +( month2 * 306 + 17 )/ 10 + date.day;
   end canonical_day_number;