let halfsecond = 500; // 500 milliseconds

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

​

  // Check every half second

  if (millis() - lastdraw >= halfsecond) {

    fill(0);

    rect(width / 2 - 25, width / 2 - 25, 50, 50); // rect

​

    lastdraw = millis(); // Update the time

  }

}

​