mongodb calculate active user x days using aggregate

sample code below is calculating 7 days active users

duration = 7; // active user 7 days

dateTo = new Date();

dateTo.setHours(0, 0, 0);

dateFrom = new Date();

dateFrom.setDate(dateTo.getDate() - duration);

dateFrom.setHours(0, 0, 0);

db.getCollection("users").aggregate( // collection users

[

{
$match: { // filter by "created"
created: { $gte: dateFrom, $lt: dateTo }
}
},

$group: {

_id: "id", // group by "id"

uniqId: { $addToSet: "$id" }, // get uniq "id" only

},

]

);