Here's a library for handling a bunch of popular I2C-based real-time clock chips: https://github.com/adafruit/Adafruit-uRTC

And here is the documentation for it: http://micropython-urtc.readthedocs.io/en/latest/

As you can guess from the naming, I was supported by Adafruit Industries to write this one (but of course it will work with any breakout boards for those chips, not just theirs). I will most likely create more libraries for them.