I have a sidebar on our intranet which lists, hierarchically, posts by category.

Such as:

    -- HR
    ---Events
    ----Picnic

However, this list has gotten long, and some categories are no longer even used.  I would like to only category links where there are posts less than a year old.

Right now, I am using the default Categories Widget in the sidebar, but I can use a different widget or hand-code it.

How would I best approach this?