ds1307 drift too big
Well, I solved the drift. After checking the crystal capacity and seeing it is correct as the frequency, I decided to power the Arduino power from an external power supply, my  DS1307 uses the +5v from the Arduino board. With the new power supply the clock is accurate ( I can't see a drift of second in 24 h ). I don't know if it was a problem in having a barely enough voltage with the USB or if that supply was "dirty", but in any case the single cause was the power supply.