Database
[{timestamp:ISODate("2020-01-28T20:42:00.000Z"),data:1,},{timestamp:ISODate("2020-01-28T18:42:00.000Z"),data:10,},{timestamp:ISODate("2020-01-28T15:42:00.000Z"),data:100,},{timestamp:ISODate("2020-01-25T15:42:00.000Z"),data:1000,},{timestamp:ISODate("2020-01-17T15:42:00.000Z"),data:10000,}]
Query
db.collection.aggregate([{$project:{_id:0,date:{$dateToString:{format:"%Y%m%d",date:"$timestamp"}}}},{$group:{_id:"$date"}}])