how to disable previous dates in CalendarExtender control through its render event?

Basically, I just want allow select dates greater than today. I'd prefer this way to avoid show alert messages.

Solution

I don't think that it is supported in the current version of the Toolkit to restrict selectable dates. This is a simple workaround handling the ClientDateSelectedChanged-Event and validate the selected date:

How to make sure user does not select a date earlier than today or greater than today

There could be instances where you do not want the user to select a day earlier than the current date. For example: when you are providing the user a form to book tickets, you would not like him to choose an earlier date. To achieve this requirement, use the following javascript code.

Prevent the User from selecting a Date Earlier than today

<head runat="server">
    <title>Calendar Extender</title>
    <script type="text/javascript">

    function checkDate(sender,args)
    {
        if (sender._selectedDate < new Date())
        {       
            alert("You cannot select a day earlier than today!");
            sender._selectedDate = new Date(); 
            // set the date back to the current date
            sender._textbox.set_Value(sender._selectedDate.format(sender._format))
         }
    }
    </script>
</head>

Call the code:

<form id="form1" runat="server">
        <asp:ScriptManager ID="ScriptManager1" runat="server" />
        <div>

            <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
            <cc1:CalendarExtender ID="CalendarExtender1"
            runat="server" OnClientDateSelectionChanged="checkDate" TargetControlID="TextBox1" />

        </div>
    </form>

Select Date Greater than today

In the javascript, just change this line sender._selectedDate > new Date() Note: You may argue that the user can still change the date by typing into the textbox or entering an invalid date. Well that can be easily handled using a ValidationControl and is covered in the next tip.

Add validation to the CalendarExtender Control

A simple way to add validation to the Calendar is to add a ValidationControl to the textbox associated with a CalendarExtender. You have two choices:

Add an Extender to the ValidationControl. To do so, drag and drop a ValidationControl > click on the smart tag of the ValidationControl > choose Add Extender. From the Extender Wizard, choose ValidatorCalloutExtender. Using this approach makes it extremely easy to discover and attach control extenders to your controls. In VS 2005, you had to do this process manually, by wiring up control extenders.
You can choose not to add the Extender. We will go ahead with option A. We will be adding two ValidationControls to the TextBox. The first, a CompareValidator to check if the user does not enter an invalid date (Eg: May 32) and second, a RangeValidator to keep the date range as desired.

Adding CompareValidator

<asp:CompareValidator ID="CompareValidator1" runat="server"
                ControlToValidate="TextBox1" Display="Dynamic" ErrorMessage="Invalid Date"
                Operator="DataTypeCheck" Type="Date">
</asp:CompareValidator>
<cc1:ValidatorCalloutExtender ID="CompareValidator1_ValidatorCalloutExtender"
                runat="server" Enabled="True" TargetControlID="CompareValidator1">
</cc1:ValidatorCalloutExtender>
Adding RangeValidator – We will restrict the user to select a date range starting from today to 15 days from now.
<asp:RangeValidator ID="RangeValidator1" runat="server"
                ControlToValidate="TextBox1" ErrorMessage="RangeValidator"
                Type="Date">
</asp:RangeValidator>
<cc1:ValidatorCalloutExtender ID="RangeValidator1_ValidatorCalloutExtender"
                runat="server" Enabled="True" TargetControlID="RangeValidator1">
</cc1:ValidatorCalloutExtender>

In the code behind of your page, add this code C#

protected void Page_Load(object sender, EventArgs e)
{
    RangeValidator1.MinimumValue = System.DateTime.Now.ToShortDateString();
    RangeValidator1.MaximumValue = System.DateTime.Now.AddDays(15).ToShortDateString();
}

VB.NET

 Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
        RangeValidator1.MinimumValue = System.DateTime.Now.ToShortDateString()
        RangeValidator1.MaximumValue = System.DateTime.Now.AddDays(15).ToShortDateString()
 End Sub

Well those were some tips associated with the CalendarExtender. As future versions of the toolkit are released, we should be hopeful that there will exist easier ways, of achieving this functionality.

From: http://www.dotnetcurry.com/ShowArticle.aspx?ID=149

Another advanced approach would be to extend the CalendarExtender javascript, but then you have your own custom version of the ajax toolkit.

http://codegoeshere.blogspot.com/2007/06/extending-calendarextender.html