What is Diachronic symmetry? We might see it in the way we tell historical narratives about something... In biology it would be a way of telling the ontogenetic story of an organisism, or the phylogenetic story of a species. In economics, it might be the emergence of relations of production. But diachronic symmetry is also something we feel. We feel it's time for change ("and now for something completely different!"), or we feel that something needs developing, or we feel that something simply has to be forced to happen. This symmetry of the flow of experience is genuinely diachronic. It's in the moment; not rational. Whereas narrative histories are constructs, emerging from the flow, they are in essence synchronic.
What's particularly interesting with Bergson is that he situates rationality with the conic sections, and not with temps