unsigned long started_time; 
int led_state;

// int: 2 bytes = 16
// 2^16 = 65536/2 = 32767 = 32 seconds
// long: 4 bytes = 32
// unsigned long = 49 days

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  // digitalWrite(LED_BUILTIN, HIGH);
  // delay(1000);
  // digitalWrite(LED_BUILTIN, LOW);
  // delay(10000);

  // Serial.println(millis());
  // delay(1000);

  int current_time = millis();
  if (current_time - started_time > 1000) {
    led_state = !led_state;
    started_time = current_time;
  }

  Serial.print(current_time);
  Serial.print(" - ");
  Serial.print(started_time);
  Serial.print(" = ");
  Serial.println(current_time - started_time);

  digitalWrite(LED_BUILTIN, led_state);
}