When editing a "member of" AR, I tried to add ???? (any other non-integer seems to do the same) as the end date (to see what happened, mainly). The result was most curious.

If I leave the start date blank and add ???? as end date, I get a "Value must be an integer" notice. All fine.
If I put a year in the start date and add ???? as end date, I get a disaster, starting with:
Caught exception in MusicBrainz::Server::Controller::Edit::Relationship->edit "Date::Calc::PP::Delta_Days(): Date::Calc::Delta_Days(): not a valid date at /home/musicbrainz/musicbrainz-server/script/../lib/MusicBrainz/Server/Form/Role/DatePeriod.pm line 27"