extract day from interval

Hi,

I'm not very savvy with interval manipulations, but I have a query like
this:

---<---------------cut here---------------start-------------->---
SELECT DISTINCT ON (table_a.id) table_a.id, table_a.var2,
EXTRACT(DAY FROM (table_b.date - table_a.date)) AS age,
FROM table_a INNER JOIN table_b
ON (table_a.id = table_b.id)
WHERE table_a.date IS NOT NULL
ORDER BY table_a.pupid, age;
---<---------------cut here---------------end---------------->---

Is this an efficient way to get the days from the time interval? Both
'date' columns are of type "Date". Thanks.

Cheers,

--
Seb