[{"_id":"1522845653126"},{"_id":"1522838153324"},{"_id":"1513421466415"},{"_id":"1515488183153"},{"_id":"1521571234500"}]

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