Convert millis() to readable time

void setup() {
  Serial.begin(9600);
  while (!Serial) { }
  Serial.println("Begin conversion from millis to readable time");
}

void loop() {
  String readableTime;
  getReadableTime(readableTime);

  Serial.println(readableTime);
  delay(5000);
}

void getReadableTime(String &readableTime) {
  unsigned long currentMillis;
  unsigned long seconds;
  unsigned long minutes;
  unsigned long hours;
  unsigned long days;

  currentMillis = millis();
  seconds = currentMillis / 1000;
  minutes = seconds / 60;
  hours = minutes / 60;
  days = hours / 24;
  currentMillis %= 1000;
  seconds %= 60;
  minutes %= 60;
  hours %= 24;

  if (days > 0) {
    readableTime = String(days) + " ";
  }

  if (hours > 0) {
    readableTime += String(hours) + ":";
  }

  if (minutes < 10) {
    readableTime += "0";
  }
  readableTime += String(minutes) + ":";

  if (seconds < 10) {
    readableTime += "0";
  }
  readableTime += String(seconds) + " ago";
}

BOARD?=esp32:esp32:t-beam
PORT?=/dev/cu.SLAB_USBtoUART

.PHONY: default lint all flash clean

default: all flash clean

lint:
	cpplint --extensions=ino --filter=-legal/copyright *.ino

all:
	arduino-cli compile --fqbn $(BOARD) ./

flash:
	arduino-cli upload -p $(PORT) --fqbn $(BOARD)

clean:
	rm -r build