export function getDay(num, str) {
  //如果要获取昨天的日期，num就是-1， 前天的就是-2，依次类推。str表示年月日间的分割方式。
  const today = new Date()
  const nowTime = today.getTime()
  const ms = 24 * 3600 * 1000 * num
  today.setTime(nowTime + ms)
  const oYear = today.getFullYear()
  let oMoth = (today.getMonth() + 1).toString()
  if (oMoth.length <= 1) oMoth = "0" + oMoth
  let oDay = today.getDate().toString()
  if (oDay.length <= 1) oDay = "0" + oDay
  return oYear + str + oMoth + str + oDay
}