Interesting coincidence... I just wrote a library for the 1307, too :-) Was trying to share mine through playground, but the registration page seems to be down. I'll look into putting it somewhere.
The main difference between my library and the other two is that I need to use the square wave output and the chip's battery-backed RAM memory (rather than burn out the EEPROM). So am providing functions for more of the nitty-gritty details of the chips capabilities.
I appreciate the mention of the Playground code; even though it's far simpler than I need, I'm still new at Arduino and C++, so models that show how to do the extern thing to set up the class for the user are helpful. (Also wondering about intializing Wire for the class object. What if other libraries work with other two-wire devices? Are we going to have scope issues for the Wire object? Multiple Wire objects in different scopes? But that's something for another thread.)
Was looking at your code, alvarojusten, saw the function to represent the day of week as a string. Don't know if you noticed, but all the clock does in increment the day of the week by one (rolling from 7 to 1) every time it hits midnight. It does not seem to have any fixed association between the date/month/year and what day of the week it is. Convenient in some ways as you are perfectly able to define 1=Sunday, 1=Monday... or 1=Thursday for that matter (if you can get the hang of Thursdays...).
Similarly, it is perfectly happy to let you set the date to Feb 30... and then it continues to the 31st. Seems to know if the current m/d/y is the last day of the month -- and if so, it will roll to the next month at midnight. But it won't force the date to be valid for the month/year.