I thought something like this
<ajaxToolkit:CalendarExtender ID="TextBox1_CalendarExtender" runat="server"
TargetControlID="txtDatumPoaganje" Format="MM/dd/yyyy" StartDate=<%=DateTime.Now%>>
But it doesnt work. I can make something similar with JavaScript and alert message:
how to disable previous dates in CalendarExtender control through its render event?
but it's not the same.