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