String to date（）函数不返回正确的ISO Date

                {
                 $addFields: {
                   date: {$substr : ["$visits.visit_date", 0,10]}
                 }
               },
                {
                    $project:{
                        now: new Date(),
                        returns: new Date("2017-09-14"),
                        visit_date: "$visits.visit_date",
                        //dateDifference: { $subtract: [ new Date(), "$visits.visit_date" ] },

                        split: "$date",
                        dates: new Date("$date")
                    }
                }
                {
                 $addFields: