[{"_id":ObjectId("5b4fa13c0ec5e844757333d5"),"payment_date":20180621,"payment_date_formatted":20180721},{"_id":ObjectId("5b4fa13c0ec5e844757333d7"),"payment_date":20180626,"payment_date_formatted":20180726}]

db.collection.aggregate([{"$match":{"payment_date":{"$gte":20170601}}},{"$addFields":{"payment_date_formatted":{"$add":["$payment_date",100]}}},{"$project":{"payment_date":{"$toLong":"$payment_date"},"payment_date_formatted":{"$toString":{"$toLong":"$payment_date_formatted"}}}}])