I think this is a routine javascript issue.
it's an easy fix to restore loading calendar.js in head. unfortunately that adds 84k to initial requests (prototype is 124k, the html is often ~7-15k range). so it's a high price for that functionality. I tried compressing it, and a couple of angles on loading it only when a date editor is requested-- no luck on either score. there is a general problem here of needing to be able to load cardtype-specific librararies at edit-time.