Well the code does seem pretty straightforward. DS1307 accuracy is governed by the crystal's accuracy, circuit layout, and temperature. Many crystals used with DS1307s have specs of ±20 or ±30 ppm. My expectation would be for the circuit to operate reasonably close to that. While I don't consider 20-30 ppm to be great accuracy, practically, it's probably OK for many applications, but represents a minimum standard IMHO.