Thanks for the review and the good suggestions. Hopefully this new patch is an improvement.

I didn't know about the context manager for assertRaises - I was just following the format for another ValueError test a few lines above.

The frozenset and re-wrapped comment were left from playing around with another way to do the checks, and I've corrected them.

I think the conditionals around calculating the julian and year are clearer now as well.

Please (obviously) let me know if there are further changes. Also please let me know if this is not the proper way to respond to the code review!