super easy button handler.cpp

byte oldState = HIGH;
time_t downMillis = 0;
time_t lastTickMillis = 0;
void loop() {
  byte state = digitalRead(BTN);
  if (oldState != state) {
    if (state == LOW) {
      clock.tick(); // button just pressed, tick the first time...
      lastTickMillis = downMillis = millis();
    }
    oldState = state;
  }

  if (state == LOW) {
    if (millis() - downMillis < 2300) {
      // within 2300ms (add 300ms buffer), tick every 1000ms.
      if (millis() - lastTickMillis > 1000) {
        clock.tick(); 
        lastTickMillis = millis();
      }
    } else {
      // after 2300ms, tick every 500ms.
      if (millis() - lastTickMillis > 500) {
        clock.tick();
        lastTickMillis = millis();
      }
    }
  }

  clock.draw(); // draw the clock
}