Set expiry time for a session field

Is there a way in Rails 3 to expire a field in the session. When I searched for this I only find ports about how to expire the hole session, which I do not want to.

In my situation I have the following:

session["END_DATE"] = @end_date

And I want to tell Rails to expire the field session["END_DATE"] after a certain amount of time which is different (a lot less) from the global expiry time for the hole session.

Thank you