code: 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
  }
docs: 返回前几天或后几天的日期
  
  @param {Date} date 日期或数字
  @param {Number} day 天(默认当天)、前几天、后几天
  @param {String} mode 获取时分秒(null默认当前时分秒)、日初(first)、日末(last)
  @return {Date}