Arduino: millis() rollover handling

// Created by James the Enginerd.
// More information at:  http://www.cmiyc.com/blog/2012/07/16/arduino-how-do-you-reset-millis/

#define interval 1000

unsigned long waitUntil=0;

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  if ((unsigned long)(millis() - waitUntil) >= interval) {  // check for rollover
    // It's time to do something!
    digitalWrite(13, !digitalRead(13));  // Toggle the LED on Pin 13
    waitUntil = waitUntil + interval;  // wait another interval cycle
  }
}