function pad(n) {
      return n.toString().length === 1 ? `0${n}` : n;
    }

    function serializeActivities(arr) {
      const obj = {};
      for (let i = 0, l = arr.length; i < l; i++) {
        const thisDate = arr[i].updatedAt;
        const day = pad(thisDate.getDate());
        const month = pad(thisDate.getMonth() + 1);
        const year = thisDate.getFullYear();
        const key = [year, day, month].join('-');
        obj[key] = obj[key] || 0;
        obj[key]++;
      }
      return obj;
    }