Technically, that's not a bug: Tolkien gave instructions for converting "Shire Calendar" dates to modern calendar dates in one of the appendices. (All hobbit months had 30 days, with some extra "holiday" dates stuck in between months at midsummer and midwinter. Leap days were added in midsummer, too.)
One source for more information is http://en.wikipedia.org/wiki/Middle-earth_calendar#Hobbit_Ca...
(Personally, I celebrate on Sept. 22 and Mar. 25 just for simplicity. I'm not sure what I'd do if an important event had fallen on Mid-year's day.)