It should be pretty straightforward, it can even be done completely in the frontend.
Something like this can be used to add classNames to discussion list items based on discussion attributes:
extend(DiscussionListItem.prototype, 'elementAttrs', function (attrs) {
if (this.attrs.discussion.createdAt()/*.doSomeComparisonWithDate()*/) {
attrs.className += ' myclassname';
}
});
Then you can customize styling with CSS with the new class name.