I forked and modified "goebish" code from a comment below.https://gist.github.com/DeanDavis/7a9a830037dddbfc4af3d70103680a8b
Does time and date just like "goebish" but reverted to the DS1307 clock from the OP project.
The calibration in this code is much easier to do. I couldn't understand how to adjust the calibration values using the "square" calibration from the OP code.
Few other minor enhancements.