[{"name":"user1","blogs":[{"title":"blog1","created":"02-01-2020"},{"title":"blog2","created":"10-01-2020"}]},{"name":"user2","blogs":[{"title":"blog3","created":"01-02-2020"},{"title":"blog4","created":"01-01-2020"}]}]

db.collection.aggregate([{$unwind:"$blogs"},{$project:{"_id":0,name:1,blogs:1,date:{$dateFromString:{dateString:"$blogs.created",}}},},{$sort:{date:-1}},{$project:{date:0}}])