AFTER the Butlerian Jihad (201-108 B.G.), for certain:
FH in the Religion Appendix of Dune wrote: Hesitantly, the leaders of religions whose followers had spilled the blood of billions began meeting to exchange views. It was a move encouraged by the Spacing Guild, which was beginning to build its monopoly over all interstellar travel, and by the Bene Gesserit who were banding the sorceresses.
The Guild is described as "embryo" and just "beginning to build" its monopoly, which firmly places the C.E.T. on the Before Guild side of the B.G./A.G. divide. But when? Was it before or after the Battle of Corrin (88 B.G.), arguably the most important political event of the immediate post-Jihad/pre-Guild period? And if after, how many years later?
The C.E.T. meets for seven years, a period which could easily fit within the twenty years before Corrin and quite luxuriously in the 88 years after. But the ways the Guild is described make me think it more likely that it took place closer to the earlier rather than the later end of things.