volatile uint64_t now = 1629881966; // current epoch time
int hour, minute, second;

void setupTimer1() {
  noInterrupts(); 
  TCCR1A = 0;  // Clear registers
  TCCR1B = 0;
  TCNT1 = 0;
  OCR1A = 15624;                        // 1 Hz (16000000/((15624+1)*1024))
  TCCR1B |= (1 << WGM12);               // CTC
  TCCR1B |= (1 << CS12) | (1 << CS10);  // Prescaler 1024  
  TIMSK1 |= (1 << OCIE1A);              // Output Compare Match A Interrupt Enable
  interrupts();
}

unsigned long currentEpochTime;

void setup() {
  Serial.begin(115200);
  setupTimer1();

  currentEpochTime = now;
}

void loop() {
  static uint32_t printTime;
  if (millis() - printTime > 1000) {
    printTime = millis();
    hour = (now / 3600) % 24;
    minute = (now / 60) % 60;
    second = now % 60; 

    Serial.print("Actual time ");
    Serial.print(hour);
    Serial.print(":");
    Serial.print(minute);
    Serial.print(":");
    Serial.println(second);

    Serial.println("");

    uint64_t mTime = currentEpochTime + millis() / 1000;
    hour = (mTime / 3600) % 24;
    minute = (mTime / 60) % 60;
    second = mTime % 60; 

    Serial.print("Millis time ");
    Serial.print(hour);
    Serial.print(":");
    Serial.print(minute);
    Serial.print(":");
    Serial.println(second);

    Serial.println("");
  }

}

// Timer1 interrupt service routine
ISR(TIMER1_COMPA_vect) {
  now++;
}