code: function getWhatWeek (date, week, day) {
    var time, whatDayTime, currentDay, customDay
    date = toStringDate(date)
    if (baseExports.isDate(date)) {
      customDay = Number(/^[0-7]$/.test(day) ? day : date.getDay())
      currentDay = date.getDay()
      time = getDateTime(date)
      whatDayTime = time + ((customDay === 0 ? 7 : customDay) - (currentDay === 0 ? 7 : currentDay)) * DAY_TIME
      if (week && !isNaN(week)) {
        whatDayTime += week * WEEK_TIME
      }
      return new Date(whatDayTime)
    }
    return date
  }
docs: 返回前几周或后几周的星期几
  
  @param {Date} date 日期
  @param {Number} week 周(默认当前周)、前几周、后几周
  @param {Number} day 星期天(默认0)、星期一(1)、星期二(2)、星期三(3)、星期四(4)、星期五(5)、星期六(6)
  @return {Date}