两中方法:
1.可以用API来做:
  [StructLayout (LayoutKind.Sequential)]
  public struct SYSTEMTIME
  {
       public short Year;
       public short Month;
       public short DayOfWeek;
       public short Day;
       public short Hour;
       public short Minute;
       public short Second;
       public short Miliseconds;
  }

  //api函数声明
  [DllImport ("kernel32.dll", CharSet=CharSet.Ansi)]
  public extern static bool SetSystemTime(ref SYSTEMTIME time);
  
  private void button1_Click(object sender, System.EventArgs e)
  {
       //调用代码
       SYSTEMTIME t = new SYSTEMTIME ();
       t.Year = 2000;
       t.Month = 1;
       t.Day = 2;
       t.Hour = 12-8; //这个函数使用的是0时区的时间,对于我们用+8时区的,时间要自己算一下.如要设12点，则为12-8
       t.Minute = 5;
       bool v = SetSystemTime(ref t);
       Console.WriteLine(v.ToString());
  }

2. VB.NET 特有的方法,很简单
    Today=日期
    TimeOfDay=时间