unsigned long prevMillis = 0;
const long interval = 1000;

void setup() {
  Serial.begin(9600);
}

void loop() {
  unsigned long now = millis();

  if (now - prevMillis >= interval) {
    unsigned long lateness = (now - prevMillis) - interval;
    prevMillis = now;

    Serial.print("Tick! Lateness: ");
    Serial.println(lateness);
  }
}