Thanks for pointing me to your solution! I now realize that there is no strict validation of the date fields in Dataverse other than that characters like the -sign are not allowed.
I'm wondering if there is a best-practice recommendation on how to represent BC dates in the ISO 8601 standard, which I guess, underlies the date fields in the Dataverse metadata schemas. The Wikipedia article on ISO 8601 says the following:
To represent years before 0000 or after 9999, the standard also permits the expansion of the year representation but only by prior agreement between the sender and the receiver. An expanded year representation [±YYYYY] must have an agreed-upon number of extra year digits beyond the four-digit minimum, and it must be prefixed with a + or − sign instead of the more common AD/BC (or CE/BCE) notation; by convention 1 BC is labelled +0000, 2 BC is labeled −0001, and so on.
Since + and - signs are not allowed in Dataverse date fields, BC seems to be the next best solution. My next question would then be: Is there a best-practice recommendation on how to represent time spans ranging from a BC year to an AD year? The heiDATA dataset indicates the dates in the following way: Start: 99BC ; End: 0550. Another variant would be Start: 99BC ; End: 550AD. Any ideas about which of these (or other alternatives) we should prefer?