Database
[{"_id":1,"title":"Harry Potter","year":2000,"author":"JK. Rowling"},{"_id":2,"title":"Harry Potter 99","year":2020,"author":"JK. Rowling"}]
Query
db.collection.aggregate([{"$match":{"author":"JK. Rowling"}},{"$group":{"_id":"$author","enddate":{$max:"$year"},"startdate":{$min:"$year"}}},{$addFields:{"total":{$subtract:["$enddate","$startdate"]}}}])