Database
[{"_id":{"$oid":"6213baaba013b7c5f1232e25"},"birthDate":239290215000,"surname":"Yang","name":"Ruby","joinDate":1534861815000,"children":[{"birthDate":876749415000,"surname":"Yang","name":"Bob"},{"birthDate":926753415000,"surname":"Yang","name":"Mel","joinDate":1579005015000}],}]
Query
db.collection.update({},[{"$set":{"birthDate":{"$toDate":"$birthDate"},"joinDate":{"$toDate":"$birthDate"},"children":{"$map":{"input":"$children","in":{"surname":"$$this.surname","name":"$$this.name","birthDate":{"$toDate":"$$this.birthDate"}}}}}}])