Re: [NOVICE] Timestamp with time zone change (error) in 7.3.2?

Doug Silver <dsilver(at)urchin(dot)com> writes:
> [ why does he get ]

test=# select '2003-04-04'::date::timestamptz;
timestamptz
------------------------
2003-04-03 23:59:00-05
(1 row)

Doug was kind enough to give me access to his machine (a FreeBSD 4.6
box) to look into it. The answer is that the timezone tables on this
machine seem to have been built with leap second information; this
causes the results of localtime() and related operations to diverge
from what Postgres is expecting.

What actually happens internally is that localtime() returns the value
2003-04-03 23:59:38-05 (22 seconds off the expected result), but we drop
the seconds part for reasons mentioned in timestamp2tm(), giving the
observed behavior. I believe that 22 seconds is about right for the
accumulated number of leap seconds since 1970, so I'm, um, leaping to
the conclusion that localtime is doing a leap-second-aware computation.

FreeBSD's "man localtime" points out

STANDARDS
The asctime(), ctime(), difftime(), gmtime(), localtime(), and mktime()
functions conform to ISO/IEC 9899:1990 (``ISO C89''), and conform to
ISO/IEC 9945-1:1996 (``POSIX.1'') provided the selected local timezone
does not contain a leap-second table (see zic(8)).

We are expecting the POSIX-specified behavior (no accounting for leap
seconds).

Not sure if there's anything much we can do about this except to document
"don't do that". It seems impractical to make our datetime arithmetic
operations cope with leap-second-aware timekeeping.

One idea that comes to mind is to test for leap-second-aware behavior
(for example, by checking to see that localtime() of a value that should be
exactly midnight is exactly midnight) and complain about it if we find
we are on a leap-second-using machine. But I'm not sure if it's worth
the trouble. I'm also not sure exactly where/when to perform this test
--- perhaps when setting a new timezone value? Comments anyone?

regards, tom lane