const int startTime = 1211;
const unsigned long startMillis = 1547;

int millisToHourMin(int startTime, unsigned long startMillis=0){
	//since millis() returns an unsigned long, it will roll over in about 90 days
	//by defining nextMillis as unsigned long also, it should roll over 1 second in advance of millis() rollover
	static unsigned long nextMillis;  
	if (nextMillis == 0){ //initializer nextMillis
		nextMillis = millis();
	}
	static int time;  //will hold the number of seconds since reset (corrected for start without reset using startMillis)
	int seconds; //hold intermediate results of calculation
	int sec; //seconds since startMillis
	int min; //minutes since startMillis
	int hrs; //hours since startMillis
  int startSeconds; //derived from startTime
	int startHours; //derived from startTime
	int startMin; //derived from startTime
	int newMin; //new minutes, corrected for startTime
	int newHours; //new hours, corrected for startTime
	static int newTime; //time to return, in format newHours * 100 + newMin
	const int SECONDS_PER_MINUTE = 60;
	const int MINUTES_PER_HOUR = 60;
	const int HOURS_PER_DAY = 24;
	const int MILLIS_PER_SECOND = 1000;
	if(millis() >= nextMillis) {
		nextMillis += MILLIS_PER_SECOND; //execute this if once per second
		time++; //number of seconds since reset
		startSeconds = startMillis / MILLIS_PER_SECOND; //allow for non-reset time setting
		seconds = time - startSeconds; // this should never go negative, corrected for time setting without reset
		sec = seconds % SECONDS_PER_MINUTE;		
		seconds /= SECONDS_PER_MINUTE;
		min = seconds % MINUTES_PER_HOUR;		
		seconds /= MINUTES_PER_HOUR;
		hrs = seconds % HOURS_PER_DAY;		
		seconds /= HOURS_PER_DAY;
    //Serial.println(time);
	
    startHours = startTime / 100; // startTime is formatted as an integer = hours * 100 + minutes, so 1:51 pm is 1351
    startMin = startTime % 100;
    newMin = min + startMin;
    if (newMin >= MINUTES_PER_HOUR){
      newMin = newMin - MINUTES_PER_HOUR;
      hrs++;
    }
    newHours = hrs + startHours;
    if (newHours >= HOURS_PER_DAY){
      newHours = newHours - HOURS_PER_DAY;
    }
    newTime = newHours * 100 + newMin;
  }
	return newTime;
}

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
  int myTime = millisToHourMin(startTime, startMillis);
  Serial.print("calculated time =");
  Serial.println(myTime);
  delay(1000);

}