[{"_id":"123","receivedTime":ISODate("2021-08-10T17:56:56.305Z"),"deliveryTime":ISODate("2021-08-10T17:56:59.941Z"),"processedTime":3.636},/** 2*/{"_id":"234","receivedTime":ISODate("2021-08-10T17:57:00.223Z"),"deliveryTime":ISODate("2021-08-10T17:57:00.691Z"),"processedTime":0.468},/** 3*/{"_id":"345","receivedTime":ISODate("2021-08-10T17:57:19.812Z"),"deliveryTime":ISODate("2021-08-10T17:57:20.331Z"),"processedTime":0.519}]

db.collection.aggregate([{$match:{"_id":{$in:["123","234"]}}},{"$project":{"receivedTime":1,"deliveryTime":1,"processedTime":{"$divide":[{"$subtract":["$deliveryTime","$receivedTime"]},1000/**1000 for sec, 60000 for minute, 3600000 for hour*/]}}}])