On a mongo 3.6, group items by month from field with timestamp value

MongoDB3-group-by-timestamp.js

db.getCollection('Flights').aggregate([
    { $match: {"airport_code" : "AMS"}}
      ,{"$group": {"_id": 
            { $dateToString: {
                date: {"$add": [new Date(0), 
                    {"$multiply": ["$created_at",1000 ]} 
                ] } ,
                format: "%Y-%m"} 
            }
    , "count": {"$sum": 1}}}
  ])