Database
[{"oldDate":ISODate("2019-05-10T07:40:23.000Z"),"newDate":ISODate("2019-05-14T05:16:27.000Z"),}]
Query
db.collection.aggregate([{"$addFields":{"timestamp":{"$subtract":["$newDate","$oldDate"]}}},{"$addFields":{"daysSpent":{"$sum":{"$divide":["$timestamp",86400000]}}}}])