Strange results with date/interval arithmetic

I am seeing strange results from date/interval computations involving
months.

I get the correct answers because I have a negative setting relative to
GMT. Here are my results with TZ=EST5EDT:

test=> select '2001/3/1'::date - '1 month'::interval;
?column?
------------------------
2001-02-01 00:00:00-05
(1 row)

With GMT it is OK too:

test=> select '2001/3/1'::date - '1 month'::interval;
?column?
------------------------
2001-02-01 00:00:00+00
(1 row)

However, with GMT+1 I see a big failure:

test=> select '2001/3/1'::date - '1 month'::interval;
?column?
------------------------
2001-01-29 00:00:00+01
(1 row)

Why does it say 2001-01-29?

This is interesting:

test=> select '2001/7/1'::date - '1 month'::interval;
?column?
------------------------
2001-05-31 00:00:00+02
(1 row)

test=> select '2001/8/1'::date - '1 month'::interval;
?column?
------------------------
2001-07-01 00:00:00+02
(1 row)

Because August and July have the same number of months, it worked. I am
going to research this but someone may know the solution already.

--
Bruce Momjian | http://candle.pha.pa.us
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