database.js

// db.episodes.findOne({})
{
	"_id" : ObjectId("5e8563a07aeda6f729a56e33"),
  "data" : {
    "date_gmt" : "2020-03-30T10:00:08"
  }
}

script.js

// I want to take the field data.date_gmt and convert it from a text object to a new Date() object.

// This one sets {created_at: "2020-03-30T10:00:08Z"}
db.episodes.updateMany({},[{$set: {created_at: "$data.date_gmt"}}])

// This one sets {created_at: ISODate("1970-01-01T00:00:00Z")}
db.episodes.updateMany({},[{$set: {created_at: (new Date("$data.date_gmt" + "Z"))}}])

// My goal is to set to: {created_at: ISODate("2020-03-30T10:00:08Z")}
db.episodes.updateMany({},[{$set: {created_at: {$dateFromString: {dateString:("$data.date_gmt")}}}}])