Database
[{timestamp:{"$date":"2022-03-15T22:11:34.711Z"},},{timestamp:{"$date":"2022-03-20T22:11:34.711Z"}}]
Query
db.collection.aggregate([{$match:{timestamp:{$gte:{"$date":"2022-03-01T00:00:00.000Z"},$lte:{"$date":"2022-03-31T23:59:59.999Z"},}}},{$project:{date:{$dateToParts:{date:"$timestamp"}},}},{$group:{_id:{date:{year:"$date.year",month:"$date.month",day:"$date.day"}},"count":{"$sum":1}}},{"$group":{"_id":null,"originData":{"$push":"$$ROOT"}}},{"$project":{"_id":0,"data":{"$concatArrays":[{"$map":{"input":{"$range":[0,30,1]},"in":{"$let":{"vars":{"date":{"$add":[{"$date":"2022-03-01T00:00:00.000Z"},{"$multiply":["$$this",86400000]}]}},"in":{"_id":{"date":{"day":{"$dayOfMonth":"$$date"},"month":{"$month":"$$date"},"year":{"$year":"$$date"}}},"count":0}}}}},"$originData"]}}},{"$unwind":"$data"},{$group:{_id:{date:{year:"$data._id.date.year",month:"$data._id.date.month",day:"$data._id.date.day"}},"count":{"$sum":"$data.count"}}},{"$sort":{"_id.date.year":1,"_id.date.month":1,"_id.date.day":1}}])