add filter for date added to collection.
you can add a string like "added:today" or "added:<1m3d" to the filter textedit.

there is no GUI for it at the moment, but this at least provides the backend support.

yes, this as a discoverability of -1000, but i need some UI ideas (and i think Dan has plans to redo the edit filter dialog completely, so i'm somewhat holding off).