I have not found a library that will drop directly into the SDK, but there are several libraries out there that you can look at for inspiration. You will also want to look at the TWI master driver for handling the I2C communication with the DS1307.
If you intend to power the nRF at all times (which is the case for most products), it could be an option to use the internal RTC instead of a external RTC. You would have to handle the complexities of date and time, such as adjusting for months with fewer than 31 days and corrections for leap year in software, but the accuracy will be just as good as using the DS1307 (as it only depends on the 32 kHz crystal).
The pin numbering can be confusing, as it is not stated explicitly. However, the pin numbers you refer to in software are always the GPIO pin numbers (P0.x in the product specification and DK marking). Have you looked at the TWI Sensor Example?
(Regarding the crystal you will anyway need a 32 kHz crystal for the DS1307, so you do not save a crystal on the BOM by using the DS1307.)
The pin numbering can be confusing, as it is not stated explicitly. However, the pin numbers you refer to in software are always the GPIO pin numbers (P0.x in the product specification and DK marking). Have you looked at the TWI Sensor Example?
(Regarding the crystal you will anyway need a 32 kHz crystal for the DS1307, so you do not save a crystal on the BOM by using the DS1307.)
