#include <Wire.h> 
#include <LiquidCrystal_I2C.h>

constexpr uint8_t startSecond = 0;
constexpr uint8_t startMinute = 17;
constexpr uint8_t startHour = 18;

LiquidCrystal_I2C lcd(0x27,16,2);

void goSecond(uint8_t mySecond) {
   if ((mySecond < 10) || (mySecond == 0)) {
      lcd.setCursor(6,0);
      lcd.print("0");
      lcd.setCursor(7,0);
      lcd.print(mySecond);
   }
   else {
     lcd.setCursor(6,0);
     lcd.print(mySecond);
   }
}

void goMinute(uint8_t myMinute) {
  if ((myMinute < 10) || (myMinute == 0)) {
      lcd.setCursor(3,0);
      lcd.print("0");
      lcd.setCursor(4,0);
      lcd.print(myMinute);
   }
   else {
     lcd.setCursor(3,0);
     lcd.print(myMinute);
   }
}

void goHour(uint8_t myHour) {
  if ((myHour < 10) || (myHour == 0)) {
      lcd.setCursor(0,0);
      lcd.print("0");
      lcd.setCursor(1,0);
      lcd.print(myHour);
   }
   else {
     lcd.setCursor(0,0);
     lcd.print(myHour);
   }
}

void setup() {
  lcd.init();                        
  lcd.backlight();
  
  goSecond(startSecond);
  goMinute(startMinute);
  goHour(startHour);
}

void loop() {
  uint32_t currentMillis = millis();
  static uint32_t interval = 1000, previousMillis = 0;
  static uint8_t mySecond = 0;
  static uint8_t myMinute = 0;
  static uint8_t myHour = 0;
  static byte flgColons = 0;

  if(currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    flgColons = !flgColons;
    if (flgColons) {
      lcd.setCursor(2,0);
      lcd.print(":");
      lcd.setCursor(5,0);
      lcd.print(":");  
    }
    else {
      lcd.setCursor(2,0);
      lcd.print(" ");
      lcd.setCursor(5,0);
      lcd.print(" ");  
    } 
    if (mySecond < 59) {
       mySecond++;
       goSecond(mySecond);
    }
    else {
      mySecond = 0;
      goSecond(mySecond);
      if (myMinute < 59) {
          myMinute++;
          goMinute(myMinute);
      }
      else {
        myMinute = 0;
        goMinute(myMinute);
        if (myHour < 23) {
          myHour++;
          goHour(myHour);
        }
        else {
          myHour = 0;
          goHour(myHour);
        }
      }   
    }
  }
}