[{"_id":1,"begin":"2020-10-01","dates":[{"name":"one","created":"2020-10-10","updated":"2020-10-11"},{"name":"two","created":"2020-10-12","updated":"2020-10-13"}]}]

db.collection.update({},[{$set:{begin:{$add:[{$toDate:"$begin"},259200000]},dates:{$map:{input:"$dates",in:{name:"$$this.name",created:{$add:[{$toDate:"$$this.created"},259200000]},updated:{$add:[{$toDate:"$$this.updated"},259200000]}}}}}}])