I would like such a database to include a list of conferences by field of study and location of the conference. As it stands several people have webpages where they post about conferences and there's also the AMS page but some conferences still slip through and I don't hear about them till they're over. One would assume that a giant database which all professional mathematicians could contribute to would not have this problem. Conference organizers could post to this database and in this way get the word out to a much larger audience. It's also not at all hard (using say SQL) to make your database self-cleaning, i.e. remove conferences that have already happened.