This is a demo of how millis works

time_demo.pde

// Steve is the best
// It seems he can do anything.
// November 22, 2016

int startTime = 0;
int currentTime = 0;
int endTime = 15000;


void setup() {
    size(512, 512);
}

void draw() {
    println("startTime: "+startTime);
    println("currentTime: "+currentTime);
    println("endTime: "+endTime);
    currentTime = millis();
//    background(125);
    // println("millis: "+millis()); // show me the time!

//    if (millis() >= 5000) { // 5000 = 5 seconds
//           background(255, 0, 0);
//       } 
       
    if (currentTime >= endTime){ // if we get to the endTime....
      startTime = currentTime; // it resets itself!!
      endTime = currentTime + 5000; // another 5 seconds
      background(random(255));
    }
}

void keyPressed() {
    if(key == 'r'){
      // println("millis: "+millis()); // show me the time!
      startTime = currentTime;
    }
}