The astronomical format of the calendar year that the Coligny calendar represents may well be far older, as calendars are usually even more conservative than rites and cults. The date of its inception is unknown, but correspondences of Insular Celtic and Continental Celtic calendars suggest that some early form may date to Proto-Celtic times, roughly 800 B.C. The Coligny calendar achieves a complex synchronisation of the solar and lunar months. Whether it does this for philosophical or practical reasons, it points to a considerable degree of sophistication.