mongodb aggregate calculated no of days between two dates code example

Example 1: mongodb date before 2 days

// 2 days before
"StartDate" : { "$lt" : new Date(ISODate().getTime() - 1000 * 86400 * 2) }

Example 2: mongodb subtract dates

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