#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27,20,4);
int i = 0;
unsigned long int lastTime = 0;
const int delayTime = 900;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  lcd.init();
  lcd.backlight();
  lcd.clear();
  lcd.setCursor(0,0);
  lcd.print("Hello World!");
  Serial.println();
}

void loop() {
  if (millis() - lastTime >= delayTime || lastTime == 0) {
    sec();
    lastTime = millis();
  }
  user_input();
}

void sec() {
  lcd.setCursor(0,0);
  lcd.print("                    ");
  lcd.setCursor(0,0);
  i = millis() / 1000;
  lcd.print(String(i));
  //delay(1000);
}

void user_input() {
  if (Serial.available()) {
    String inputstr = Serial.readStringUntil('\n');
    lcd.clear();
    sec();
    lcd.setCursor(0,1);
    lcd.print(inputstr);
  }
}