[{"id":1,"createdAt":new Date("2021-10-29T08:00:00Z")},{"id":2,"createdAt":new Date("2021-10-29T08:15:00Z")},{"id":3,"createdAt":new Date("2021-10-30T06:00:00Z")},{"id":4,"createdAt":new Date("2021-10-30T08:12:00Z")},{"id":5,"createdAt":new Date("2021-10-30T09:00:00Z")}]

db.collection.find({$expr:{$eq:[{$hour:"$createdAt"},8]}})