volatile unsigned long lastButtonPress = millis();

// Debouncing na timerze:
// https://github.com/khoih-prog/TimerInterrupt/blob/master/examples/SwitchDebounce/SwitchDebounce.ino
void initButton() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  // Order of ISRs matter: RISING should be invoked first
  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN), buttonISRtime, CHANGE);
  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN), buttonISRstate, RISING);  
}

void buttonISRstate() {
  if ((millis() - lastButtonPress) > 100) {
    if (countdown < 0)
      countdown = 0;
    else if (countdown == 0)
      countdown = COUNTDOWN;
    else {
      countdown = -1;
      digitalWrite(BUTTON_LED_PIN, LOW);
    }
  }
}

void buttonISRtime() {
  lastButtonPress = millis();
}