Comment 1 for bug 554113

That's because the "ephem" library thinks that "2010-03" means the "negative 3" month of 2010 — the third-from-the-last-month of 2009, in other words. You can see this by constructing plain Date objects:

>>> ephem.Date('2010/03')
2010/3/1 00:00:00
>>> ephem.Date('2010-03')
2009/9/1 00:00:00

Are you asking for "-" to become a new date separator character? So far PyEphem has just supported "/", like the astronomy library it is built upon, and so we would have to think through the implications of changing it. I wonder if anybody has ever used the negative-number functionality? I wasn't even sure, until I saw this report, that I myself knew that "2010-03" was even a valid PyEphem date! :-)