如何在当前DateTime.Now中添加一天然后删除一天？

Question

首先我做： 在Form1的顶部：

[DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        internal static extern bool SetLocalTime(ref SYSTEMTIME lpSystemTime);

        [StructLayout(LayoutKind.Sequential)]
        internal struct SYSTEMTIME
        {
            public ushort wYear;
            public ushort wMonth;
            public ushort wDayOfWeek;    // ignored for the SetLocalTime function
            public ushort wDay;
            public ushort wHour;
            public ushort wMinute;
            public ushort wSecond;
            public ushort wMilliseconds;
        }

        private int day;
        private int month;
        private int year;
        private int hour;
        private int minute;

然后我有两种方法可以提前一天在系统中移动时间，另一种方法是将时间移回当天的当天：

private void ChangeTimeForword()
        {
            dt = DateTime.Now.AddDays(1);
            day = dt.Day;
            month = dt.Month;
            year = dt.Year;
            hour = dt.Hour;
            minute = dt.Minute;
            SYSTEMTIME time = new SYSTEMTIME();
            time.wDay = (ushort)day;
            time.wMonth = (ushort)month;
            time.wYear = (ushort)year;
            time.wHour = (ushort)hour;
            time.wMinute = (ushort)minute;

            if (!SetLocalTime(ref time))
            {
                // The native function call failed, so throw an exception
                throw new Win32Exception(Marshal.GetLastWin32Error());
            }
        }


private void ChangeTimeOriginal()
        {
            try
            {
                dt = DateTime.Now.AddDays(-1);
                day = dt.Day;
                month = dt.Month;
                year = dt.Year;
                hour = dt.Hour;
                minute = dt.Minute;
                SYSTEMTIME time = new SYSTEMTIME();
                time.wDay = (ushort)day;
                time.wMonth = (ushort)month;
                time.wYear = (ushort)year;
                time.wHour = (ushort)hour;
                time.wMinute = (ushort)minute;

                if (!SetLocalTime(ref time))
                {
                    // The native function call failed, so throw an exception
                    throw new Win32Exception(Marshal.GetLastWin32Error());
                }
            }
            catch (Exception err)
            {
                MessageBox.Show("Error" + err.ToString());
            }
        }

所以它第一次起作用但是日期改变了。 首先，日期是9/30/2013 7:43 PM 然后它现在改为日期：2013年6月11日，它在6月12日和6月11日之间移动

奇怪。那是为什么？