extract(epoch from infinity) is not 0

Hello,

=# select extract(epoch from 'infinity'::timestamp);
date_part
-----------
0

A better value would be 'infinity'::float8. Ditto for -infinity.

I'm trying to use a box-based index to represent the intervals in a
table containing a pair of fields date_from, date_to (timestamps),
where semi-open intervals are represented with +/- infinity. Building
the boxes using extract(epoch from ...) creates wrong entries as
semi-open intervals are converted into a box with a corner in (0,0).

-- Daniele