If you ever experience problems with a DS1307 check the following:

If you don't have a Backup Battery connected to the Vbat Pin pull that Pin LOW.

With Vbat floating the DS1307 might be put into low current mode and will not recognize any data input.

I have realized this on a few of my DS1307s and it has taken me quite some time time to find out what the problem was.

regards

Ralph