docs: function getWhatDay (date, day, mode) {
    date = toStringDate(date)
    if (baseExports.isDate(date) && !isNaN(day)) {
      date.setDate(date.getDate() + Number(day))
      if (mode === STRING_FIRST) {
        return new Date(_dateFullYear(date), _dateMonth(date), date.getDate())
      } else if (mode === STRING_LAST) {
        return new Date(getDateTime(getWhatDay(date, 1, STRING_FIRST)) - 1)
      }
    }
    return date
  }