Casting dates

Hi,

Does anyone know if it's possible to calculate the number of days (or
weeks, seconds, minutes, or what-have-you) in a given interval without
casting to EPOCH and performing manipulations there?

PostgreSQL usually returns intervals created by subtraction in days
and smaller fragments, I would like to know if it's possible for it to
return years. (Integer division of the number of days by 365 would
produce an almost-correct result, but it's rather inelegant.)

Subtracting two timestamps gives a day count:

cal=> select timestamp 'today' - timestamp 'may 1, 2000';
?column?
----------
727 days
(1 row)

Subtracting two days-only intervals gives the expected result:

cal=> select interval '6000 days' - interval '100 days';
?column?
-----------
5900 days
(1 row)

But it doesn't work for all like-unit intervals: some are reported
using other quantities:

cal=> select interval '6000 minutes' - interval '100 minutes';
?column?
--------------
4 days 02:20 <--- not the expected '5900 minutes'
(1 row)

And most notably, it doesn't work for years, where it would be most useful:

cal=> select timestamp 'jan 24, 1998 00:00' - timestamp 'jan 24, 1990 00:00';
?column?
-----------
2922 days
(1 row)

cal=> select extract (years from timestamp 'jan 24, 1998 00:00' - timestamp 'jan 24, 1990 00:00');
date_part
-----------
0
(1 row)

My aim is to make it easier to write a function that manipulates years
(the code fragment in question takes two timestamps, A and B, and an
integer N, and subtracts A from B to see if they differ by a multiple
of N years. If not, then it adds years to B to ensure that A and B
differ by a multiple of N).

Any ideas would be appreciated.

Cordially,

Joe Barillari