control in web page fails if only in GAC

I'm trying to put a DateTimePicker on a web page, using
<OBJECT classid="System.Windows.Forms.dll#
System.Windows.Forms.DateTimePicker" etc.>

(without the line break after #) but it just puts up an
empty box with a scrollbar, indicating some kind of
failure. With some experimenting I've found that, despite
what everyone says, this sort of thing never succeeds in
using an assembly from the client's GAC, but works fine
if the assembly is in the same directory with the web
page. So, I tried copying System.Windows.Forms.dll from
the GAC into the web directory (ugh!), but then I just
got an image icon and it still didn't work. The examples
everyone gives are always of a derived class, never the
standard controls. Am I missing something obvious, or is
this basic functionality broken?