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.
Try SelectedDate instead of StartDate.
Also, the link below says you can't set selecteddate from the html side, instead you must do it from code-behind.
I haven't confirm this however. http://forums.asp.net/t/1293771.aspx/1