How Do I Validate A Textbox To Show Error Msg If Date Entered Is In The Past

txtDate.Text = Calendar1.SelectedDate.ToString("yyyy/MM/dd");
string Duedate = txtDate.Text;
string date = (DateTime.Now).ToString("yyyy/MM/dd");

if ((txtDate.Text != "") && (Convert.ToDateTime(Duedate) >= Convert.ToDateTime(date)) )
{
do some coding for future date here
}
else
{
lblErrMsg.text =" the entered date is in the past ";
}

txtDate.Text = Calendar1.SelectedDate.ToString("yyyy/MM/dd");
if(string.isNullOrwhitespace(txtDate.Text)
{
   lblErrMsg.text =" You must enter a date. ";
} else {

    if (Calendar1.SelectedDate >= DateTime.Now)
    {
        // do some coding for future date here
    }
    else
    {
        lblErrMsg.text =" the entered date is in the past ";
    }
}

string str = "12/10/2014"; // in your case your calendar control
string strFormat = "dd/MM/yyyy";
DateTime x;
DateTime y;

TryParseDate(str,strFormat,out x) 
TryParseDate(DateTimeNow.ToString("dd/MM/yyyy"),strFormat,out y))


if(x > y)
{
lblErrMsg.text =" the entered date greater then current date" ;

}

private bool TryParseDate(string str,string strFormat, out DateTime result)
{
    if(!String.IsNullOrEmpty(str))
     {
          return DateTime.TryParseExact(str,strFormat,new CultureInfo("en-US"),DateTimeStyles.None,out result)
     }
    else
     {
          result = new DateTime(); return false;
     }


}