using System;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("本年度第一天：{0}", GetYearFirstDate(DateTime.Now));

            Console.Read();
        }

        /// <summary>  
        /// 獲取本年年初, 本年度第一天
        /// </summary>  
        /// <param name="dateTime">當前時間 DateTime.Now or DateTime.UtcNow</param>  
        /// <returns>后面的具體時、分、秒和傳入值相等</returns>  
        public static DateTime GetYearFirstDate(DateTime dateTime)
        {
            // 本年年初
            return dateTime.AddMonths(1 - dateTime.Month).AddDays(1 - dateTime.Day);
        }
    }
}