将输入的日期转为上个月的今天
lastMonth(a) {
let selectDate = a.split('-')//["2021", "05", "17 00:00"]
let selectYear = selectDate[0]
let selectMonth = selectDate[1]
let last1m = this.getLast1m(selectMonth, selectYear).last1m
let year2 = this.getLast1m(selectMonth, selectYear).year
//数组填充
let arr1 = []
arr1.push(year2)
arr1.push