I was just reading, in a book entitled 'Early Christian Ireland', about how the early church in Ireland experienced difficulty in setting the date of Easter. They needed to use a "luni-solar calendar" (surprise, surprise) and had a wee bit of bother setting the rules. The main difficulty was the fact that 12 moon months was 11 days short of a solar year. The Stone Age astronomers had the same problem to face, but they seem to have had it all worked out:

http://www.mythicalireland.com/astronomy/moonmovements/metoniccycle.html