[{"_id":ObjectId("611a11a09500700009d09a12"),"userId":"abc","createdAt":ISODate("2021-08-14T07:22:00.535Z")},{"_id":ObjectId("611a11a09500700009d09a13"),"userId":"abc","createdAt":ISODate("2021-08-15T07:20:00.535Z")},{"_id":ObjectId("611a11a09500700009d09a14"),"userId":"def","createdAt":ISODate("2021-08-15T07:22:00.535Z")},{"_id":ObjectId("611a11a09500700009d09a15"),"userId":"abc","createdAt":ISODate("2021-08-16T07:18:00.535Z")},{"_id":ObjectId("611a11a09500700009d09a16"),"userId":"abc","createdAt":ISODate("2021-08-16T07:20:00.535Z")},{"_id":ObjectId("611a11a09500700009d09a17"),"userId":"def","createdAt":ISODate("2021-08-16T07:22:00.535Z")}]

db.collection.aggregate([{"$match":{"createdAt":{"$gte":ISODate("2021-08-15T00:00:00.000Z")}}},{"$sort":{"createdAt":-1}},{"$group":{"_id":"$userId","createdAt":{"$first":"$createdAt"}}}])