compteur de secondes...

void setup()
{
  Serial.begin( 115200 );
}

void loop()
{
  uint32_t t1 = millis();
  static uint32_t t2 = t1;
  static uint32_t secondes = 0;

  if ( t1 - t2 >= 1000 )
  {
    t2 = t1;
    ++secondes;
    Serial.println( secondes );
  }
}