I actually looked at supporting dates like this, but if you go through the Met's open dataset (https://github.com/metmuseum/openaccess) that kind of "alternative calendar with no reference to the BCE/CE dates" is basically nonexistent.

There are references to the Islamic and Japanese calendar systems, but always next to the CE equivalent.

Data entry is fortunately being done by modern people, so the translation to CE/BCE is usually baked in, and all you need to support is every possible way somebody could say "the early half of" and "5th millenium B.C. to mid 1914"