One of the quickest ways to achieve this is to use a 3rd party client-side datePicker such as that from JQueryUI.
http://jqueryui.com/datepicker/
You will also need JQuery itself: http://jquery.com/download/
You would add textboxes to your document:
<asp:textbox: id="startDate" runat="server" cssclass="datepicker-field"/>
You can then add javascript to state that you would like all textboxes with a particular css class to become datepickers:
<script>
$(document).ready(function(){
$('.datepicker-field').datepicker();
});
</script>
After your form posts-back you will be able to get the selected date value as a string from startDate.Text
So you will have to parse and validate the value