I'm also interested in this and want to know what, exactly, 'Last Updated' means?
i.e. under 'Your Listings', where this is shown, there's no clear indication that it refers to the calendar being updated.
In my opinion, if you have 'done something'... anything... on your listing, it proves that you're maintaining it - whether you add a photo, change some text or update the calendar etc.
So yesterday I made a change to our guidebook, but the updated date has not changed.
Seems a bit pointless trying to update anything else... given the comments from you guys.
Hope Airbnb can shed some light on this?