AddMonths 增加月份，有许多神奇的case需要处理，例如 1月31日 - 2月28日, 3月31日 - 4月30日。 总的逻辑为，到达下一个月的同一天，若无效前推至有效

time.go

package main

import (
	"fmt"
	"testing"
	"time"
)

// WHY：
// 

// NextMonths plus or minus natural month; many magical cases, such as 0131 - 0228, 0331 - 0430.
// reach the same day of the next month, reset the last month if it is invalid

func NextMonths(t time.Time, n int) time.Time {
	y, m, d := t.Date()
	hour, min, sec := t.Clock()

	next := time.Date(y, m+time.Month(n), d, hour, min, sec, 0, t.Location())
	if (next.Year()-y)*12+int(next.Month()-m) == n {
		return next
	}
	return time.Date(y, m+time.Month(n+1), 0, hour, min, sec, 0, t.Location())
}

func mustParseTime(s string) time.Time {
	t, _ := time.Parse("20060102", s)
	return t
}

func TestNextMonths(t *testing.T) {
	type args struct {
		start time.Time
		n     int
	}
	tests := []struct {
		name string
		args args
		want time.Time
	}{
		{"0130-0228", args{mustParseTime("20190130"), 1}, mustParseTime("20190228")},
		{"0131-0228", args{mustParseTime("20190131"), 1}, mustParseTime("20190228")},
		{"1231-0229", args{mustParseTime("20191231"), 2}, mustParseTime("20200229")},
		{"0101-0101", args{mustParseTime("20190101"), 0}, mustParseTime("20190101")},
		{"0101-0201", args{mustParseTime("20190101"), 1}, mustParseTime("20190201")},
		{"0331-0430", args{mustParseTime("20190331"), 1}, mustParseTime("20190430")},
		{"0731-0831", args{mustParseTime("20190731"), 13}, mustParseTime("20200831")},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			if got := NextMonths(tt.args.start, tt.args.n); got != tt.want {
				t.Errorf("NextMonths() = %v, want %v", got, tt.want)
			}
		})
	}
}

func main() {

	// 20190131 -> 20190228
	fmt.Println(NextMonths(mustParseTime("20190130"), 1))

	// 20190331 -> 20190430
	fmt.Println(NextMonths(mustParseTime("20190331"), 1))
}