Real-time clock on an Arduino Uno

// Real-time clock: a DS1307 keeps the date and time, and the Uno prints it.
//
//   DS1307 SDA -> A4    DS1307 SCL -> A5    VCC -> 5V    GND -> GND
//
// The Uno forgets the time whenever it resets. The DS1307 does not: it counts
// seconds on its own 32.768 kHz crystal, and on a real module a coin cell
// keeps it counting while everything else is unplugged. The Uno only asks it
// for the time over I2C, the two-wire bus on A4 and A5.
//
// RTClib does the asking. If the clock was ever stopped, the sketch starts it
// again from the date and time in setup(): put today's there before you
// upload it to a real module.

#include <Wire.h>
#include <RTClib.h>

RTC_DS1307 rtc;
uint8_t lastSecond = 99;

void print2(uint8_t value) {
  if (value < 10) Serial.print('0');
  Serial.print(value);
}

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: DS1307 real-time clock");
  if (!rtc.begin()) {
    Serial.println("no clock found: check SDA and SCL");
    while (true) {
    }
  }
  if (!rtc.isrunning()) {
    Serial.println("clock was stopped: setting it");
    rtc.adjust(DateTime(2026, 9, 23, 12, 0, 0));  // year, month, day, hour, minute, second
  }
}

void loop() {
  DateTime now = rtc.now();
  if (now.second() == lastSecond) return;
  lastSecond = now.second();
  Serial.print(now.year());
  Serial.print('-');
  print2(now.month());
  Serial.print('-');
  print2(now.day());
  Serial.print(' ');
  print2(now.hour());
  Serial.print(':');
  print2(now.minute());
  Serial.print(':');
  print2(now.second());
  Serial.println();
}