[{"_id":"612fa439gdb04331d6ea1e5d","name":"imran","createdAt":"2021-09-01T16:03:05.266Z",},{"_id":"612fa439ddb44331d6ea1e5d","name":"Hossain","createdAt":"2021-08-31T16:03:05.266Z",},{"_id":"612fa439ddb44331d6ea1e5e","name":"Alex","createdAt":"2021-08-31T16:05:05.266Z",}]

db.collection.aggregate([{$addFields:{createdAtDate:{$toDate:"$createdAt"},}},{$group:{_id:{$dateToString:{format:"%Y-%m-%d",date:"$createdAtDate"}},count:{$sum:1}}},{$project:{count:1,date:"$_id",_id:0}}])