Comment on timezone and interval types

Recently there has been some discussion about attaching a timezone to
a timestamp and some other discussion about including a 'day' part
in the interval type. These two features impact each other, since
if you add a 'day' to a timestamp the result can depend on what timezone
the timestamp is supposed to be in. It probably makes more sense to use
a timezone associated with the timestamp than say the timezone GUC or the
fixed timezone UTC.