void loop() {
  int washingTime = millis();
  unsigned long allSeconds = washingTime / 1000;
  int runHours = allSeconds / 3600;
  int secsRemaining = allSeconds % 3600;
  int runMinutes = secsRemaining / 60;
  int runSeconds = secsRemaining % 60;

  char buf[21];
  sprintf(buf, "Koniec prania: %02d:%02d:%02d", runHours, runMinutes, runSeconds);
  Serial.println(buf);
  delay(1000);
}