#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);

int long time = 0;
byte num;

void setup() {
  lcd.init();
  lcd.backlight();
  Serial.begin(9600);

  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), button, FALLING);

  lcd.print("0 sec");
}

void loop() {
  if(num == 1){
    time = millis();
    lcd.home();
    lcd.print(String(time/1000) + " sec");
  }
  if(num == 0){
    lcd.home();
    lcd.print(String(time/1000) + " sec");
  }
}

void button() {
  if(num == 0) num = 1;
  else if(num == 1) num = 0;
}