ST Function Block to Return Day of the Week

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Logic that returns the day of the week as a number (0 = Sunday to 6 = Saturday).

FUNCTION_BLOCK DayOfTheWeek

VAR_INPUT
  InDate : DATE_AND_TIME;
END_VAR

VAR_OUTPUT
  DayOfWeek : INT;
END_VAR

VAR
  Year : INT;
  cYear : INT;
  Month : INT;
  Day : INT;
  CP : INT;
  YP : INT;
  MP : INT;
  LeapYear : BOOL := FALSE;
  Valid :BOOL;
END_VAR

  Year := EXTRACT_DT_YEAR(InDate);
  cYear := STRING_TO_INT(RIGHT(INT_TO_STRING(Year),2));
  Month := EXTRACT_DT_MONTH(InDate);
  Day := EXTRACT_DT_DAY(InDate);
  Valid := TRUE;

  CASE Year OF
    1752..1799 : CP := 4;
    1800..1899 : CP := 2;
    1900..1999 : CP := 0;
    2000..2099 : CP := 6;
  ELSE
    Valid := FALSE;
  END_CASE;

  YP := cYear / 4;

  IF (MOD(cYear,400) = 0) THEN
    LeapYear := TRUE;
  ELSIF (MOD(cYear,100) = 0) THEN
    LeapYear := FALSE;
  ELSIF (MOD(cYear,4) = 0) THEN
    LeapYear := TRUE;
  ELSE
    LeapYear := FALSE;
  END_IF;

  CASE Month OF
    1 : 
      IF LeapYear THEN
        MP := 6;
      ELSE
        MP := 0;
      END_IF;
    2 :  
      IF LeapYear THEN
        MP := 2;
      ELSE
        MP := 3;
      END_IF;
    3 : MP := 3;
    4 : MP := 6;
    5 : MP := 1;
    6 : MP := 4;
    7 : MP := 6;
    8 : MP := 2;
    9 : MP := 5;
    10 : MP := 0;
    11 : MP := 3;
    12 : MP := 5;
  END_CASE;

  IF Valid THEN
    DayOfWeek := MOD(CP + cYear + YP + MP + Day, 7 );
  ELSE
    DayOfWeek := -1;
  END_IF;

END_FUNCTION_BLOCK

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