[{"_id":ObjectId("4c69d19532f73ad544000001"),"content":"blah blah blah","comments":[{"author":"jim","content":"comment content 1","date":"07-24-1995"},{"author":"joe","content":"comment content 2","date":"07-24-1996"},{"author":"amy","content":"comment content 3","date":"09-10-1999"}]},{"_id":ObjectId("4c69d19532f73ad544000002"),"content":"blah blah blah","comments":[{"author":"jim","content":"comment content 1","date":"07-24-1995"},{"author":"joe","content":"comment content 2","date":"07-24-1996"},{"author":"amy","content":"comment content 3","date":"07-24-1997"}]}]

db.collection.aggregate([{$unwind:"$comments"},{$addFields:{"comments.dateISO":{$dateFromString:{dateString:"$comments.date",format:"%m-%d-%Y"}}}},{$sort:{"comments.dateISO":-1}},{$group:{_id:"$_id",comment:{$first:"$comment"},comments:{$push:"$comments"}}}])