overlaps() does not work as expected?

I've quite some trouble with the overlaps function:
SELECT overlaps('9.6.2005'::date, '9.6.2005'::date, '9.6.2005'::date,
'9.6.2005'::date);
returns true (these are german timestamps dd.mm.yyyy)

SELECT overlaps('8.6.2005'::date, '9.6.2005'::date, '9.6.2005'::date,
'9.6.2005'::date);
returns false

Is this a bug or a (quite strange) feature?

Thanks!