Sadly, I think the only way this is stable longterm is if the moon is tidally locked, which means 1 month = 1 day.
@indi Somehow I knew neptune had to have a solution to this :)
@starkatt It's actually tuned to a precise 1 day/year precession, because multi-year cycles can be nice too. (extremely Neptunian voice) Y'don't want to get TOO predictable, y'knowdude? ;)
@starkatt Or you cheat using an artificial moon made of gravity-magic: http://wiki.postfurry.net/wiki/Neptunian_Calendar ;)