mongodb aggregate subtract two fields from different collections code example

Example 1: difference between dates mongodb

db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ "$$NOW", "$date" ] } } } ] )

Example 2: mongodb subtract dates

db.sales.aggregate( [ { $project: { item: 1, total: { $subtract: [ { $add: [ "$price", "$fee" ] }, "$discount" ] } } } ] )