Timer without delay()

Processing Coding Questions
1

I have written a code for a stopwatch (when button pressed, it starts), but I have used delay(). Is there a way to perform it without delay()?

lcd.setCursor(0, 1);
  if (buttonValue1 == LOW) {
    startTime = millis();
    timerMode++;
    delay(400);
  }
  if (timerMode == 1) {
    lcd.setCursor(0, 1);
    lcd.print("Time : ");
    lcd.setCursor(7, 1);
    lcd.print((millis() - startTime) / 1000.0);
    lcd.setCursor(12, 1);
    lcd.print(" sec");
  }
  if (timerMode > 1) {
    delay(6000);
    timerMode = 0;
    lcd.clear();
    lcd.setCursor(3, 0);
    lcd.print("TIMER");
2

Adding a stopwatch to a clock