Using VWD2010 Express, how to set calender controls

I've got a calender named dateCalendar
I'm trying to make a code which means that when someone presses a date before todays date a message comes up saying 'Please enter a valid date'.
I've tried        
 if (dateCalendar.SelectedDate <= DateTime.Now.Date)
        {
            Response.Write("<script>alert('Please enter a valid date')</script>");
        }
But it doesn't seem to work, and I'd rather not use Javascript. Could anyone help please?
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