Getting the amount of overlap when using OVERLAPS

Given 2 date ranges, the overlaps functions returns TRUE or FALSE. I
want to find the # of days that are overlapping. Is there a "built in"
way to do this?  Should I just write a function to do it?

For instance, this example overlaps, but I want to know how much does
it overlap?

SELECT (DATE '2001-02-16', DATE '2001-12-21') OVERLAPS
       (DATE '2001-10-30', DATE '2002-10-30');

Thanks in advance!
Tony Wasson