time math - Bug or expected behavior?

I traced a bug in our application down to this basic operation:

set timezone to 'US/Eastern';

select '11/02/2008'::timestamptz, '12:10'::time,
'11/02/2008'::timestamptz + '12:10'::time;

I have a date and a time stored separately and I want to combine them,
and use them in some timezone-aware calculations. When I add the time
12:10 to the date 11/2/08, I expect the timestamp "11/2/08 12:10" but
instead, I get "11/2/08 11:10".

It's probably not coincidence that daylight saving time rolls back one hour
on the morning of 11/2. Still, I would have expected the above behavior
when adding an interval to a timestamp, but not a time. Is the time being
cast to an interval before the add? Is there a better way to combine a
date with a time and get a timestamptz ? (the values are stored in the database, and are not literals as in my example)