// library imports

// global variables

void setup() {
  Serial.begin(9600);
  Serial.println("Initializing...");

  // setup
}

void loop() {
  Serial.println("Running...");

  // local main event loop variables

  uint32_t lastMillis = millis();
  while (true) {
    uint32_t currentMillis = millis();
    if (currentMillis < lastMillis) {
      // skip main loop in case of millis() overflow
      continue;
    }

    // main event loop actions

    lastMillis = currentMillis;
  }
}