[{_id:"mongoIdA",createdAt:ISODate("2021-03-08T00:59:00.000Z")},{_id:"mongoIdB",createdAt:ISODate("2021-03-09T00:00:02.000Z")},{_id:"mongoIdC",createdAt:ISODate("2021-03-09T00:00:10.000Z")},{_id:"mongoIdD",createdAt:ISODate("2021-03-09T00:00:14.999Z")},{_id:"mongoIdE",createdAt:ISODate("2021-03-09T00:00:15.000Z")},{_id:"mongoIdF",createdAt:ISODate("2021-03-09T00:00:29.999Z")},{_id:"mongoIdG",createdAt:ISODate("2021-03-09T05:01:12.000Z")},{_id:"mongoIdH",createdAt:ISODate("2021-03-09T05:01:14.999Z")},]

db.collection.aggregate([{"$addFields":{"time":{"$toLong":"$createdAt"}}},{$group:{_id:{"$floor":{"$divide":["$time",15000]}},itemCount:{$sum:1},items:{$push:"$_id"}}},{$addFields:{date:{$convert:{input:{"$multiply":["$_id",15000]},to:"date"}}}},{$sort:{date:1}}])