LINQ Sort on Interface Property

LINQ Sort on Interface Property

Two of my Entities implement the IDatedItem interface. I'd like to union,
then sort these, for enumerating as a single list. I must retain
entity-specific properties at enumeration-time.
To clarify by example, one approach I tried was:
Context.Table1.Cast<IDatedItem>().
Union(Context.Table2.Cast<IDatedItem>()).
SortBy(i => i.Date).
ToList();
In trying to do this, I've run into various errors.
LINQ to Entities only supports casting EDM primitive or enumeration types.
Unable to process the type '.IDatedItem[]', no known mapping to the value
layer
etc.
Details:
The entity-specific properties will be rendered in order in a web page.
In a parallel feature, they will be serialized to a JSON result hierarchy.
The IDatedItem interface here is a simplification of the actual shared
properties.
In practice, the tables are filtered before the union.
I'd like to be able to perform LINQ aggregate operations on the results as
well.