Delays via millis wraparound test

void outputCountdown(char *lbl,
    unsigned long currMillis,
    unsigned long prevMillis,
    unsigned long interval) {


  Serial.print('\n');Serial.println(lbl);
  Serial.print("Count from: ");
  Serial.print(prevMillis);
  Serial.print(", interval: ");
  Serial.println(interval);
  Serial.print("target millis: ");
  Serial.println(prevMillis + interval);

  unsigned long elapsed;
  do {
    currMillis++;
    elapsed = currMillis - prevMillis;
    Serial.print("curr: ");
    Serial.print(currMillis);
    Serial.print(", elapsed: ");
    Serial.print(elapsed);
    Serial.println(elapsed >= interval ? " Expired" : "...");
    delay(100);
  } while (elapsed < interval);
}

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

    // Parameters: lable, currentMillis, previous millis, interval);
  outputCountdown("wrap: ", 0xFFFFFFF8, 0xFFFFFFF8, 16);
  outputCountdown("plain: ", 25, 25, 10);
}

void loop() {
  // put your main code here, to run repeatedly:

}