Database
[{"code":[{"a":"2016-05-10","b":{"12":1,"00":2,"06":3}},{"a":"2017-12-31","b":{"12":4,"00":5,"06":6}}]}]
Query
db.collection.aggregate([{$set:{code:{$arrayToObject:{$reduce:{input:"$code",initialValue:[],in:{$concatArrays:["$$value",{$map:{input:{$objectToArray:"$$this.b"},as:"b",in:{k:{$concat:["$$this.a","T","$$b.k",":00:00:0Z"]},v:"$$b.v"}}}]}}}}}}])