I apologize for the misunderstanding Michael.
Changing the editable options at run-time is unsupported in the Telerik Extensions for ASP.NET MVC. You can try achieving similar behavior using the Grid's edit event
handler instead. Once the even it triggered you can check the dates and if the conditions are met - call the cancelChanges method
I also would like to remind you that as of June 2013 the product is no longer supported and that migrating to Kendo UI
is highly recommended.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.