Real Time Clock (DS3231 RTC)

#include<Wire.h>
#include "RTClib.h"
RTC_DS3231 rtc;


unsigned long previousMillis = 0;        // will store last time LED was updated
const long interval = 1000;

void setup() {
  Serial.begin(9600);
  // put your setup code here, to run once:
  if (! rtc.begin()) {
    Serial.println(F("Couldn't find RTC"));
    while (1);
  }
  if (rtc.lostPower()) {
    Serial.println(F("RTC lost power, lets set the time!"));
    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  }

}

void loop()
{
  // put your main code here, to run repeatedly:
  DateTime now = rtc.now();

  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval)
  {
    previousMillis = currentMillis;
    Serial.print(now.year(), DEC);
    Serial.print( "/" );
    Serial.print(now.month(), DEC);
    Serial.print( "/" );
    Serial.print(now.day(), DEC),
    Serial.print( " " );
    Serial.print(now.hour(), DEC);
    Serial.print( ":" );
    Serial.print(now.minute(), DEC);
  }
}