code: function remainDays (date, term) {
    var to = this.to(date, term);
    return to ? this.diff(date, to, 'd') + 1 : null;
  }
docs: 指定した期間の残日数
  @method remainDays
  @param  {DATE}   date
  @param  {String} term  既定値: day
  @return {Number} days