Hi drobosson,
Welcome to Visual WebGui
Thanks for reporting this. The control should support it like WinForms does, so I opened a new issue on the subject which you can track here.
While this is not supported in the control, you have four options: a) Wait until we fix this, although at this point I don't have a version or a date for you, b) Create your own custom control to implement it, which would require you to go down to xslt and JavaScript, c) Find an ASP.NET control that does what you want and wrap it with the ASP.NET wrapper, or d) Create a UserControl to implement it in Visual WebGui code.
Personally, I would settle for a) although all dates are shown, then the control will validate and only allow dates between MinDate and MaxDate.
Hope this helps,
Palli