The distance calculation ignored the time zone, so the result of (b-a)
might have ended negative even if (b > a). Fixed by considering the time
zone difference.

Reported-by: Jaime Casanova
Discussion: https://postgr.es/m/CAJKUy5jLZFLCxyxfT%3DMfK5mtPfSzHA1rVLowR-j4RRsFVvKm7A%40mail.gmail.com