c# 输入某年某月某日,判断这一天是这一年的第几天?

using System;
using System.Collections.Generic;
using System.Text;
 
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("输入年份:");
            string year = Console.ReadLine();
            int iyear = int.Parse(year);
 
            Console.Write("输入月份:");
            string month = Console.ReadLine();
            int imonth = int.Parse(month);
 
            Console.Write("输入日期:");
            string day = Console.ReadLine();
            int iday = int.Parse(day);
 
            int day_in_year = 0;
 
 
 
            for (int i = 1; i < imonth; i++)
            {
                day_in_year = day_in_year + DateTime.DaysInMonth(iyear, i);
            }
 
            day_in_year = day_in_year + iday;
 
            Console.WriteLine("输入的日期是{1}年份的第{0}天", day_in_year.ToString(),year);
 
            Console.ReadKey();
        }
    }
}

DateTime d = new DateTime(2014,2,28);
Console.WriteLine(d.DayOfYear);

结果：59

string a = "2014-1-18";
Console.WriteLine(Convert.ToDateTime(a).DayOfYear);