请教 c# 时间处理问题

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleTest
{
    class Program
    {
        static void Main(string[] args)
        {
            int y = 2006, m = 12, d = 32;
            Console.WriteLine("{0}-{1}-{2}", y, m, d);

            fun(ref y, ref m, ref d);

            Console.WriteLine("{0}-{1}-{2}", y, m, d);

            Console.ReadKey();
        }

        //使用一个方法来变换年月日
        private static void fun(ref int year,ref int month,ref int day)
        {
            int[] days = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };  //每月的天数

            if(year%400==0||(year%4==0&&year%100!=0))  //闰年2月29天
            {
                days[1] = 29;
            }

            if (day <= days[month - 1]) return;  //天数不超过当月的天数，直接返回
            
            if(month==12)  //如果是12月，则要转到明年的1月
            {
                year += 1;
                month = 1;
                day -= 31;
            }
            else  //一般的情况
            {
                day -= days[month - 1];
                month += 1;
            }
        }
    }
}