#include <TimerOne.h>
#include <LiquidCrystal_I2C.h>

typedef struct
{
  unsigned char time;
  unsigned char second;
  unsigned char minute;
  unsigned char hour;
}DATETIME_STR;

LiquidCrystal_I2C lcd(0x27,16,2);
DATETIME_STR dateTime;

void setup() {
  lcd.init();
  lcd.backlight();
  
  Timer1.initialize(1000000);
  Timer1.attachInterrupt(timerCallback);
  Timer1.start();

  dateTime.hour   = 12;
  dateTime.minute = 0;
  dateTime.second = 23;
  // put your setup code here, to run once:
}

void loop() {

  if(dateTime.time)
    dateTime.time = showDateTime();

  // put your main code here, to run repeatedly:

}

void timerCallback(void)
{
  dateTime.time = 1;
  if(++dateTime.second >= 60)
  {
    dateTime.second = 0;
    if(++dateTime.minute >= 60)
    {
      dateTime.minute = 0;
      if(++dateTime.hour >= 24)
      {
        dateTime.hour = 0;
      }
    }
  }
}

unsigned int showDateTime(void)
{
  char time[16];
  lcd.setCursor(0,0);
  sprintf(time,"time: %02d:%02d:%02d",dateTime.hour,dateTime.minute,dateTime.second);
  lcd.print(time);

  return 0;
}