中国标准时间转日期YYYY-MM-DD

function switchDate(standardTime) {
  if (standardTime != "") {
    let a = standardTime;
    var Month = a.getMonth() + 1;
    var MonthDate;
    var DayDate = a.getDate();
    var Day;
    if (Month < 10) {
      MonthDate = '0' + Month;
    } else {
      MonthDate = Month;
    }
    if (DayDate < 10) {
      Day = '0' + DayDate;
    } else {
      Day = DayDate;
    }
    var datetime = a.getFullYear() + "-" + MonthDate + "-" + Day;
    console.log(datetime)
    return datetime
  }
}

中国标准时间standardTime =  Sat Jan 01 1994 00:00:00 GMT+0800 (中国标准时间) 转之后是datetime = 1994-01-01