[{trackingHistory:[{status:"Shipped",time:ISODate("2022-11-22T06:30:49.000Z")},{status:"Delivered",time:ISODate("2022-11-25T15:30:00.000Z")}]},{trackingHistory:[{status:"Shipped",time:ISODate("2022-11-22T09:29:45.000Z")},{status:"Delivered",time:ISODate("2022-11-23T19:26:00.000Z")}]}]

db.collection.aggregate([{$project:{trackingHistory:"$trackingHistory.time",_id:0}},{$group:{_id:0,timeDiff:{$push:{$abs:{$dateDiff:{startDate:{$first:"$trackingHistory"},endDate:{$last:"$trackingHistory"},unit:"hour"}}}}}},{$project:{averageHour:{$avg:"$timeDiff"},_id:0}}])