It looks, from http://codereview.musicbrainz.org/r/508/diff/ , like someone tried this, using "
100-xx-xx to -1-xx-xx" to represent BC years. However, while that does generate a "Caught exception in MusicBrainz::Server::Controller::Artist>edit "Date::Calc::PP::Delta_Days(): Date::Calc::Delta_Days(): not a valid date at /home/mbserver/musicbrainz/mb_server/script/../lib/MusicBrainz/Server/Form/Role/DatePeriod.pm line 24"" error, the bug fix described in that code review fixes the wrong thing.
BC years are not properly expressed using negative year numbers, but rather by using positive integers. (eg: 100 years before 1 AD was not -100 AD, but rather 100 BC). We likely have very few artists/etc that would ever use BC years, but currently, if you try to set "100 to 1" as the years, you get a "The end date must occur on or after the begin date" error; the only real "fix" I can see which would make sense would be the addition of a "AD/BC" dropdown next to date fields, with AD as the default.