SharePoint:DateTimeControl Access denied

In one of my project's implementations the SharePoint DateTimeControl has stopped working altogether and users with "Contribute" access were getting access denied to the field. In other words, the Calendar control was not opening, see below

It was referred like this in my application page

<SharePoint:DateTimeControl ID="dtselDate" runat="server" DateOnly="true" IsRequiredField="true" CssClassTextBox="ms-input" />

The Problem Background:

Actually DateTimeControl field uses the iframe calender page to render the actual calender. The location of iframe.aspx is C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS

The problem appeared to be with managed paths in the site collections, the Users with "Contribute access" were accessing the following path:

http://myserver/_layouts/iframe.aspx (Path to Root site collection where user doesn't have access)

However, the actual path should be http://myserver/sites/sitecollection/_layouts/iframe.aspx (Path to actual site collection where user HAVE valid access)

The solution:

The solution is pretty simple i.e. to set DatePickerFrameUrl to use the magic tokens $SPUrl SiteCollection (see below), which will ensure Contributers will access the right Site Collection.

DatePickerFrameUrl = "<% $SPUrl:~SiteCollection/_layouts/iframe.aspx %>"  DateOnly="true" runat="server" />

~site/ is an equivalent to SPContext.Current.Web.ServerRelativeUrl

~sitecollection/  is an equivalent to SPContext.Current.Site.ServerRelativeUrl

Reference

http://falakmahmood.blogspot.in/2010/05/sharepointdatetimecontrol-access-denied.html#.Ugt_mpIwfWI
http://chayadigital.wordpress.com/2011/10/04/error-with-spurl-tokens/