the timer is active when clickStartedMillis is > 0, and stopped when it is -1

TimerWithLazyGui.pde

import com.krab.lazy.*;

LazyGui gui;

long clickStartedMillis = -1;

String currentTime;
String endTime = "";

PFont bigFont, smallFont;

color blue = color(78, 135, 163);
color grey = color(126, 156, 171);


void setup() {
  size(450, 400, P2D);

  smallFont = createFont("Comic Sans MS", 20);
  bigFont = createFont("Comic Sans MS", 50);
  textFont(bigFont);

  gui = new LazyGui(this, new LazyGuiSettings().setHideBuiltInFolders(true));
}


void draw() {
  background(16);
  if (gui.button("start timer")) {
    startTimer();
  }
  if (gui.button("stop timer")) {
    stopTimer();
  }
  fill(blue);

  currentTime = elapsedTimePrettyFormat();


  text(currentTime, 100, 250);
  fill(grey);
  text(endTime, 100, 350);
}

void startTimer() {
  clickStartedMillis = millis();
}

void stopTimer() {
  endTime = elapsedTimePrettyFormat();
  clickStartedMillis = -1;
}

String elapsedTimePrettyFormat() {
  if (clickStartedMillis < 0) {
    return "00:00:00";
  }
  long elapsedTime = millis() - clickStartedMillis;
  return nf((elapsedTime / (1000*60*60)) % 24, 2, 0) + ":" +
    nf((elapsedTime / (1000*60)) % 60, 2, 0) + ":" +
    nf((elapsedTime / 1000) % 60, 2, 0);
}