int weekDay(
  int day,
  int month,
  int year
){
  // Returns the day of the week for the given date, assumed valid and
  // in the Gregorian calendar.  The returned value is: 0 for Sunday,
  // 1 for Monday, ..., 6 for Saturday; or, if the preprocessor symbol
  // MONDAY_IS_0 is defined: 0 for Monday, 1 for Tuesday, ..., 6 for
  // Sunday.

  int result;

  if (month < 3) {
    month += 12;
    --year;
  }

  result = day + (13 * month - 27)/5 + year + year/4
           - year/100 + year/400;

#ifdef MONDAY_IS_0
  result += 6;
#endif // MONDAY_IS_0

  return (result % 7);
}