So a negative date is a BC date, at least as far as extended ISO date formats are concerned.
What we might want to do for end users is display that as “123 BCE” instead of “-123”.
For the entity editing pages, if we don’t want the user to have to enter a negative number in front of the year, I think that would translate to having a checkbox for BCE dates.
I think you could do this in MB but I’m not sure it’s possible right now - it might be the old values just were added long enough ago. That said, for BookBrainz this was considered while developing it (more books than music are BC, after all) so it should not be an issue.