startDate 开始日期
enDate    结束日期 

function getDaysBetween(startDate, enDate) {
  const sDate = Date.parse(startDate)
  const eDate = Date.parse(enDate)
  if (sDate > eDate) return 0    //开始日期大于结束日期，返回0
  if (sDate === eDate) return 1  //如果日期相同 返回一天 
  return (eDate - sDate) / (1 * 24 * 60 * 60 * 1000)
}

console.log(getDaysBetween('2022-02-21','2022-02-28'));//7