User.aggregate([
  {
    /* Filter out users who have not yet subscribed */
    $match: {
      /* "joined" is an ISODate field */
      'subscription.joined': {$ne: null}
    }
  },
  {
    /* group by year and month of the subscription event */
    $group: {
      _id: {
        year: {
          $year: '$subscription.joined'
        },
        month: {
          $month: '$subscription.joined'
        }
      },
    }
  },
  {
    /* sort descending (latest subscriptions first) */
    $sort: {
      '_id.year': -1,
      '_id.month': -1
    }
  },
  {
    $limit: 100,
  },
])