Database
[{"_id":ObjectId("656e59214247fdc6cb90136d"),"title":"Caught","year":1900,"cast":["Undefined"],"genres":["Undefined"]},{"_id":ObjectId("656e59214247fdc6cb90136e"),"title":"Caught2","year":1920,"cast":["Undefined"],"genres":["Undefined"]},{"_id":ObjectId("656e59214247fdc6cb90136f"),"title":"Caught3","year":1940,"cast":["Undefined"],"genres":["Undefined"]}]
Query
db.collection.aggregate([{"$group":{"_id":null,"maxYear":{"$max":"$year"}}},{"$project":{"_id":null,"minYear":{"$subtract":["$maxYear",20]}}}])