Ancient Slavic Calendar?

I was reading a book on Russian folk beliefs where the author
mentioned an alleged 4th Century AD calendar discovered near Kiev, as
discussed by the Russian auhtor B.A. Rybakov (I do not have the
source at the moment, however).
Are any listmembers aware of such a calendar? What is the oldest
known Slavic calendar? I already know the Old Church Slavonic month
names - but do we have anything earlier?
- Chris Gwinn