using System;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("本月第一天：{0}", GetMonthFirstDate(DateTime.Now));

            Console.Read();
        }

        /// <summary>
        /// 獲取本月月初, 本月第一天
        /// </summary>
        /// <param name="dateTime">當(dāng)前時(shí)間 DateTime.Now or DateTime.UtcNow</param>  
        /// <returns>后面的具體時(shí)、分、秒和傳入值相等</returns>
        public static DateTime GetMonthFirstDate(DateTime dateTime)
        {
            // 本月月初
            return dateTime.AddDays(1 - dateTime.Day);
        }
    }
}