_STRUCTURED PROGRAMMING COLUMN_
by Jeff Duntemann

[LISTING ONE]

FUNCTION Modulus(X,Y : Integer) : Integer;

VAR
  R : Real;

BEGIN
  R := X/Y;
  IF R < 0 THEN
    Modulus := X-(Y*Trunc(R-1))
  ELSE
    Modulus := X-(Y*Trunc(R));
END;


[LISTING TWO]

PROGRAM ZelTest2;  { From DDJ 11/90 }

CONST
  DayStrings : ARRAY[0..6] OF STRING =
  ('Sunday','Monday','Tuesday','Wednesday','Thursday','Friday','Saturday');

VAR
  Month, Day, Year : Integer;

{ This function implements true modulus, rather than }
{ the remainder function as implemented in MOD.      }

FUNCTION Modulus(X,Y : Integer) : Integer;

VAR
  R : Real;

BEGIN
  R := X/Y;
  IF R < 0 THEN
    Modulus := X-(Y*Trunc(R-1))
  ELSE
    Modulus := X-(Y*Trunc(R));
END;

FUNCTION CalcDayOfWeek(Year,Month,Day : Integer) : Integer;

VAR
  Century,Holder : Integer;

BEGIN
  { First test for error conditions on input values: }
  IF (Year < 0)  OR
     (Month < 1) OR (Month > 12) OR
     (Day < 1)   OR (Day > 31) THEN
     CalcDayOfWeek := -1  { Return -1 to indicate an error }
  ELSE
    { Do the Zeller's Congruence calculation as Zeller himself }
    { described it in "Acta Mathematica" #7, Stockhold, 1887.  }
    BEGIN
      { First we separate out the year and the century figures: }
      Century := Year DIV 100;
      Year    := Year MOD 100;
      { Next we adjust the month such that March remains month #3, }
      {  but that January and February are months #13 and #14,     }
      {  *but of the previous year*: }
      IF Month < 3 THEN
        BEGIN
          Inc(Month,12);
          IF Year > 0 THEN Dec(Year,1)      { The year before 2000 is }
            ELSE                            { 1999, not 20-1...       }
              BEGIN
                Year := 99;
                Dec(Century);
              END
        END;

      { Here's Zeller's seminal black magic: }
      Holder := Day;                        { Start with the day of month }
      Holder := Holder + (((Month+1) * 26) DIV 10); { Calc the increment }
      Holder := Holder + Year;              { Add in the year }
      Holder := Holder + (Year DIV 4);      { Correct for leap years  }
      Holder := Holder + (Century DIV 4);   { Correct for century years }
      Holder := Holder - Century - Century; { DON'T KNOW WHY HE DID THIS! }

      Holder := Modulus(Holder,7);          { Take Holder modulus 7  }

      { Here we "wrap" Saturday around to be the last day: }
      IF Holder  = 0 THEN Holder := 7;

      { Zeller kept the Sunday = 1 origin; computer weenies prefer to }
      { start everything with 0, so here's a 20th century kludge:     }
      Dec(Holder);

      CalcDayOfWeek := Holder;  { Return the end product! }
    END;
END;

BEGIN
  Write('Month (1-12): '); Readln(Month);
  Write('Day   (1-31): '); Readln(Day);
  Write('Year        : '); Readln(Year);
  Writeln('The day of the week is ',
           DayStrings[CalcDayOfWeek(Year,Month,Day)]);
  Readln;
END.