I would find this discussion easier to follow if the Wikidata identifiers for the various classes and properties were mentioned, and there were pointers to relevant documentation.
The only Wikidata class or property that I could easily find is Q205892. It's discussion page, https://www.wikidata.org/wiki/Talk:Q205892, mentions a bit about conversion, but nothing about this issue.
The page segment that is supposed to be being used for discussion, https://www.wikidata.org/wiki/Wikidata:Project_chat#calendar_model_screwup, does not have any pointers to any classes, properties, or documentation.
Even the very nice email from Markus that gives numbers does not provide any information on where the numbers come from.
Please