Arduino Day of Year Function

toDayOfYear.ino

/*
  toDayOfYear
  
  This sketch demonstrates a function to convert the year, month, day
  format to the day of the year. Leap years are corrected. Unit tests
  include leap year and regular year tests for every month as well as
  corner case and error case testing.
  
  Created 30 March 2015
  Modified 14 April 2015
  J.R. Leeman
  
  www.github.com/jrleeman
  
 "THE BEER-WARE LICENSE" (Revision 42):
 <[email protected]> wrote this file.  As long as you retain this notice you
 can do whatever you want with this stuff. If we meet some day, and you think
 this stuff is worth it, you can buy me a beer in return. 
 
 */
  

void setup() {
  Serial.begin(115200);
}

void loop() {
  
  int result = 0;
  
  Serial.print("\n\n");
  Serial.println("TEST FOR NON-LEAP YEAR");
  Serial.println("RESULT, EXPECTED VALUE");
  
  result = calculateDayOfYear(20,1,2015);
  Serial.print(result);
  Serial.println(", 20");
  
  result = calculateDayOfYear(1,2,2015);
  Serial.print(result);
  Serial.println(", 32");
  
  result = calculateDayOfYear(31,3,2015);
  Serial.print(result);
  Serial.println(", 90");
  
  result = calculateDayOfYear(30,4,2015);
  Serial.print(result);
  Serial.println(", 120");
  
  result = calculateDayOfYear(5,5,2015);
  Serial.print(result);
  Serial.println(", 125");
  
  result = calculateDayOfYear(8,6,2015);
  Serial.print(result);
  Serial.println(", 159");
  
  result = calculateDayOfYear(10,7,2015);
  Serial.print(result);
  Serial.println(", 191");
  
  result = calculateDayOfYear(31,8,2015);
  Serial.print(result);
  Serial.println(", 243");
  
  result = calculateDayOfYear(29,9,2015);
  Serial.print(result);
  Serial.println(", 272");
  
  result = calculateDayOfYear(5,10,2015);
  Serial.print(result);
  Serial.println(", 278");
  
  result = calculateDayOfYear(30,11,2015);
  Serial.print(result);
  Serial.println(", 334");
  
  result = calculateDayOfYear(29,12,2015);
  Serial.print(result);
  Serial.println(", 363");
  
  Serial.print("\n\n");
  Serial.println("TEST FOR LEAP YEAR");
  Serial.println("RESULT, EXPECTED VALUE");
  
  result = calculateDayOfYear(20,1,2016);
  Serial.print(result);
  Serial.println(", 20");
  
  result = calculateDayOfYear(1,2,2016);
  Serial.print(result);
  Serial.println(", 32");
  
  result = calculateDayOfYear(31,3,2016);
  Serial.print(result);
  Serial.println(", 91");
  
  result = calculateDayOfYear(30,4,2016);
  Serial.print(result);
  Serial.println(", 121");
  
  result = calculateDayOfYear(5,5,2016);
  Serial.print(result);
  Serial.println(", 126");
  
  result = calculateDayOfYear(8,6,2016);
  Serial.print(result);
  Serial.println(", 160");
  
  result = calculateDayOfYear(10,7,2016);
  Serial.print(result);
  Serial.println(", 192");
  
  result = calculateDayOfYear(31,8,2016);
  Serial.print(result);
  Serial.println(", 244");
  
  result = calculateDayOfYear(29,9,2016);
  Serial.print(result);
  Serial.println(", 273");
  
  result = calculateDayOfYear(5,10,2016);
  Serial.print(result);
  Serial.println(", 279");
  
  result = calculateDayOfYear(30,11,2016);
  Serial.print(result);
  Serial.println(", 335");
  
  result = calculateDayOfYear(29,12,2016);
  Serial.print(result);
  Serial.println(", 364");
 
  Serial.print("\n\n");
  Serial.println("TEST FOR CORNER/ERROR CASES");
  Serial.println("RESULT, EXPECTED VALUE");
  
  result = calculateDayOfYear(0,1,2016);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(32,8,2016);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(29,2,2016);
  Serial.print(result);
  Serial.println(", 60");
  
  result = calculateDayOfYear(29,2,2015);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(31,12,2015);
  Serial.print(result);
  Serial.println(", 365");
  
  result = calculateDayOfYear(31,12,2016);
  Serial.print(result);
  Serial.println(", 366");
  
  result = calculateDayOfYear(32,12,2015);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(32,12,2016);
  Serial.print(result);
  Serial.println(", 999");
  
  Serial.print("\n\n");
  Serial.println("VERIFY LEAP YEAR CALCULATION");
  Serial.println("RESULT, EXPECTED VALUE");
  
  result = calculateDayOfYear(29,2,1700);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(29,2,1800);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(29,2,1900);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(29,2,2100);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(29,2,2200);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(29,2,2300);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(29,2,2500);
  Serial.print(result);
  Serial.println(", 999");
  
  result = calculateDayOfYear(29,2,2400);
  Serial.print(result);
  Serial.println(", 60");
  
  result = calculateDayOfYear(29,2,1600);
  Serial.print(result);
  Serial.println(", 60");
  
  result = calculateDayOfYear(29,2,2000);
  Serial.print(result);
  Serial.println(", 60");

  Serial.println("------------------ END TESTS ------------------");
  delay(10000); 
}

int calculateDayOfYear(int day, int month, int year) {
  
  // Given a day, month, and year (4 digit), returns 
  // the day of year. Errors return 999.
  
  int daysInMonth[] = {31,28,31,30,31,30,31,31,30,31,30,31};
  
  // Verify we got a 4-digit year
  if (year < 1000) {
    return 999;
  }
  
  // Check if it is a leap year, this is confusing business
  // See: https://support.microsoft.com/en-us/kb/214019
  if (year%4  == 0) {
    if (year%100 != 0) {
      daysInMonth[1] = 29;
    }
    else {
      if (year%400 == 0) {
        daysInMonth[1] = 29;
      }
    }
   }

  // Make sure we are on a valid day of the month
  if (day < 1) 
  {
    return 999;
  } else if (day > daysInMonth[month-1]) {
    return 999;
  }
  
  int doy = 0;
  for (int i = 0; i < month - 1; i++) {
    doy += daysInMonth[i];
  }
  
  doy += day;
  return doy;
}

I love the function you to get the day of the year but I ran into one issue. When using the RTCLib and you pass in now.day(),now.month(),now.year() it does not return the DOY it returns 999 and I cannot figure out why. Any idea?