#include <LiquidCrystal.h>

// RS, E, D4, D5, D6, D7
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

unsigned long previousMillis = 0;
const unsigned long interval = 1000;

int seconds = 0;
int minutes = 0;
int hours   = 0;

void setup() {
  lcd.begin(16, 2);
  lcd.print("Digital Clock: ");
}

void loop() {
  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;

    seconds++;

    if (seconds >= 60) {
      seconds = 0;
      minutes++;
    }

    if (minutes >= 60) {
      minutes = 0;
      hours++;
    }

    if (hours >= 24) {
      hours = 0;
    }

    updateDisplay();
  }
}

void updateDisplay() {
  lcd.setCursor(0, 1);

  if (hours < 10) lcd.print("0");
  lcd.print(hours);
  lcd.print(":");

  if (minutes < 10) lcd.print("0");
  lcd.print(minutes);
  lcd.print(":");

  if (seconds < 10) lcd.print("0");
  lcd.print(seconds);
}