This is actually something I'm surprised wasn't there from the start. It's very easy for me (and I assume others) to join a game jam a week or month in advance eagerly, get caught up in work or other issues, and forget to add the jam to their plans when it sneaks up.
I think in addition to showing what jams you've currently joined, a quick "Add to Google Calendar/iCal" button by them, as well as a link to the Jam page of course, would be immensely helpful, as it'd give most people a quick way to add it to their schedule!