Database
[{"Id_Cust":"4145","firstName":"Albade","lastName":"Maazou","gender":"female","date":"031981"},{"Id_Cust":"5296","firstName":"Rafael","lastName":"Oliveira","gender":"male","date":"061987"},{"Id_Cust":"6192","firstName":"Abdul Rahman","lastName":"Budjana","gender":"male","date":"011990"}]
Query
db.collection.aggregate([{$addFields:{date:{"$dateFromParts":{year:{"$toInt":{"$substr":["$date",2,4]}},month:{$toInt:{$substr:["$date",0,2]}}}}}}])