Cross domain request from JavaScript filter on DateTime field.

I am issuing a cross domain request from JavaScript.  The request should filter a standard EventCalendar based on the Start and End times.  Based on this my url looks like this

_api/web/lists/GetByTitle('My%20Event%20Cal')/Items?$filter=EndDate eq datetime'2013-08-02T18:30:00Z'

When this request is executed I get the following error

Microsoft.SharePoint.SPException: The field 'EndDate' of type 'DateTime' cannot be used in the query filter expression

True enough when I create a custom DateTime column and filter on that it works fine.  How do I fix this, I want to filter based on the standard event calendar, I dont want users to create a custom EventCalendar just for my filtering purposes.