怎样改变系统时间？？？？？？？？？求救

[ StructLayout( LayoutKind.Sequential )]
public class SystemTime 
{
public ushort year; 
public ushort month; 
public ushort dayOfWeek; 
public ushort day; 
public ushort hour; 
public ushort minute; 
public ushort second; 
public ushort milliseconds;  }
[ DllImport( "Kernel32.dll" )]
public static extern void GetSystemTime( [In,Out] SystemTime st );
[ DllImport( "Kernel32.dll" )]
public static extern bool SetSystemTime( [In,Out] SystemTime st );
SystemTime st=new SystemTime ();
GetSystemTime(st);
string s=st.year+"-"+st.month+"-"+st.day+"  "+(st.hour+8)+":"+st.minute+":"+st.second+"  "+st.dayOfWeek; st.year +=1;
bool result=SetSystemTime(st);