[{"name":"firstupdate","starttime":ISODate("2021-06-30T07:38:06.926Z"),"endtime":ISODate("2021-06-30T12:35:08.265Z"),},{"name":"secondupdate","starttime":ISODate("2021-07-29T07:41:06.926Z"),"endtime":ISODate("2021-07-30T01:52:07.937Z"),},{"name":"thirdupdate","starttime":ISODate("2021-07-15T07:43:06.926Z"),"endtime":ISODate("2021-07-14T10:34:13.269Z"),},{"name":"fourthupdate","starttime":ISODate("2021-07-21T05:11:23.654Z"),"endtime":ISODate("2021-07-21T09:46:33.000Z"),},{"name":"fifthupdate","starttime":ISODate("2021-07-07T02:34:06.926Z"),"endtime":ISODate("2021-07-07T08:11:06.926Z"),}]

db.collection.aggregate([{$group:{_id:null,diff:{$avg:{$dateDiff:{startDate:"$starttime",endDate:"$endtime",unit:"millisecond"}}}}},{$set:{diff:{$dateToString:{date:{$toDate:"$diff"},format:"%H:%M:%S"}}}}])