If you are using Peter Blum's Date Package and getting the dreaded 'Page is loading...' dialog box after a callback in an Update Panel containing date controls, be sure that the first UpdatePanel you are registering with his AjaxManager has its UpdateMode set to 'Always'. This IS documented and I overlooked this bit.
Unfortunately, the DatePackage and Validation (VAM) package AjaxManagers operate differently so it took me a bit to figure out how to work with the Date Package. It's a bit clumsier. I am hoping the next full version of his controls will have Ajax handling baked in rather than having it feel like an unwanted guest at dinner.