The background for this algorithm is as follows:
Step 3, below, uses CALGO 199 (this link is to a PDF file which requires the Adobe Acrobat Reader) from "Collected Algorithms from CACM".
1. Determine if year is 2 or 4digits and correct if necessary. Ideally, the Year will be 4digit, otherwise possible year range will be restricted to 1920 to 2019 using the 'cutoff' value shown.
cutoff = 20 ' Arbitary pivot year (1920) if year < 100 then ' year only consists of last 2digits if year < cutoff then year = 2000 + year else ' year >= cutoff year = 1900 + year end if end if
2. Determine if year is a leap year or not, after converting to 4digits.
Leap Year = False ' Assume not a Leap Year to start Factor = 0 ' Factor is amount to add to Day Index to ' give Calendar Number assuming leap year ' result remains 'False' if right (year, 2) = 0 then ' If century year and divisible by 400 if (left (year, 2) mod 4) = 0 then Leap Year = True Factor = 7 end if else ' If not a century year and divisible by 4 if (right (year, 2) mod 4) = 0 then Leap Year = True Factor = 7 end if end if
3. Determine the Day of the Week using one of the following methods:
a. Calculate Julian Day Number using CALGO 199 (JDAY) setting Year = Input Year; Month = 01; and, Day = 01 as we want the day on which the first day of the year occurs Calculate Remainder Plus 1 using formula: (Julian MOD 7) + 1 Determine Day of Week for result and adjust such that Sun = 1, Mon = 2, ..., Sat = 7 We want the numeric value only for later and Sunday must equal day #1 b. Use ANSI COBOL 89 Instrinsic Function (see examples below) to obtain the Julian day number instead of CALGO 199 (JDAY) and then proceed as for method 'a' above MOVE FUNCTION INTEGEROFDATE (yyyy0101) TO receivingfield MOVE FUNCTION INTEGEROFDAY (yyyy001) TO receivingfield Base date for ANSI Intrinsic Date Functions is 01JAN1601 which was a Monday which means that 1601 used calendar #2.
4. Determine the Calendar Number from the obtained data as follows:
if Leap Year = False then Calendar Number = (Corrected) DayOfWeek Number else Calendar Number = (Corrected) DayOfWeek Number + Factor end if

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