ds1307 incorrect timing
The best comparison clock to use to validate your DS1307 circuit is a mains-powered one - the utility takes great pains to ensure that the line frequency is very stable. A GPS clock would be a good choice, as would a local radio station that broadcasts time.

As for your DS1307, if you look up application note 58 it makes mention of specific load capacitors and crystals. You should make sure that you're using the correct load capacitors with the crystal you have. If you're not sure, you can try using something in the range of values specified in the table and re-evaluate your error.