#include <RTClib.h>
RTC_DS1307 rtc;

char daysOfTheWeek[7][12] = {
  "SUN", "MON", "TUe", "WED", "THU", "FRI", "SAT"
};

int secondOld; // store "now.second"

void setup () {
  Serial.begin(115200);
  rtc.begin(); // returns zero if RTC not found

  // set RTC to date & time sketch was compiled
  // rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));

  // set RTC manually
  // rtc.adjust(DateTime(2021, 1, 21, 3, 0, 0)); // yyyy mm dd hh:mm:ss
}

void loop () {
  DateTime now = rtc.now();
  if (secondOld != now.second()) { // only print when seconds change
    zeropad(now.year(), "/"); // format the value and add a delimeter
    zeropad(now.month(), "/");
    zeropad(now.day(), " (");
    Serial.print(daysOfTheWeek[now.dayOfTheWeek()]);
    Serial.print(") ");
    zeropad(now.hour(), ":");
    zeropad(now.minute(), ":");
    zeropad(now.second(), " ");

    secondOld = now.second(); // store old "seconds"
    Serial.println();
  }
}

void zeropad(int value, char symbol[]) {
  if (value < 10)
    Serial.print("0"); // pad the units value
  Serial.print(value); // print the value (year, month, day, hour, minute, second)
  Serial.print(symbol); // print a delimiter symbol
}