void rtc_setCG(byte val);
const byte rtc_id = 0x68; //DS1307 i2c id: 0x68h, 104 dec, 150 octal, 1101000b
pinMode(2, INPUT_PULLUP);
attachInterrupt (digitalPinToInterrupt(2), rtcout_int, FALLING);

void rtcout_int ()
{ char buf[5] = "    ";
  time_t now;
  struct tm *ts;

  wdt_reset();
  noInterrupts();
  epoque++; //this is a global volatile unsigned long variable that holds the number of seconds since the arduino epoque
  interrupts();
}
void rtc_setCG(byte val)
{ Wire.beginTransmission(rtc_id);
  Wire.write((byte)0x07);
  Wire.write(val);
  Wire.endTransmission();
}
ISR (TIMER1_COMPA_vect)
{ byte nsecond;
  nsecond=(byte)((millis()/1000) % 60);
  if ( nsecond != second )
  { second=nsecond;
    epoque++;
    minute=(byte)((epoque / 60) % 60);
    hour=(byte)(((epoque / 3600) + timezone) % 24);
  }
}
rtc_setCG(0x10); //this is how I set up the ds1307 to do 1hz on the SQW/OUT