void setup() {
  Serial.begin(115200);
  Serial.println("delta T world!");
}

unsigned long oldTime;

unsigned long CalculateDeltaTime() {
  unsigned long TimeNow = millis();
  unsigned long deltaTime = TimeNow - oldTime;

  oldTime = TimeNow;

  return deltaTime;
}

int counter;

void loop() {

  unsigned long deltaTime = CalculateDeltaTime();
  
  Serial.print(counter); counter++;
  Serial.print("        ");
  Serial.println(deltaTime);

  int delayTime = random(100, 1000);

  Serial.print("                         going to sleep for ");
  Serial.println(delayTime);
  delay(delayTime);
}