Re: BUG #16797: EXTRACT(EPOCH FROM timestamp) is not using local timezone

That phrasing makes sense.

You're correct, it is a nominal value, even if there might be a perception
that the value aligns with assuming UTC.

It's definitely tricky.  Prior to this thread, I would have expected the
following to be the equivalent.  Now I'd prefer that the first wasn't even
an allowed operation without an explicit cast - but that ship has likely
sailed long ago.

# SET TIME ZONE 'EST5EDT'; select extract (epoch from '2020-03-09
00:00:00'::timestamp - '2020-03-08 0:00:00'::timestamptz);
 date_part
-----------
     82800
(1 row)

# SET TIME ZONE 'EST5EDT'; select extract (epoch from '2020-03-09
00:00:00'::timestamp) - extract (epoch from '2020-03-08
0:00:00'::timestamptz);
 ?column?
----------
    68400

regards,
-dana

On Fri, Jan 1, 2021 at 12:18 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Dana Burd <djburd@gmail.com> writes:
> > Can I suggest a slight alteration in the (9.9.1. EXTRACT) epoch
> > documentation to help others:
>
> > "For timestamp with time zone values, the number of seconds since
> > 1970-01-01 00:00:00 UTC (can be negative); for date and timestamp values,
> > the number of seconds since 1970-01-01 00:00:00 UTC (date and timestamp
> > will assume UTC regardless of local timezone in order to maintain
> > immutability - one may explicitly cast timestamp to timestamptz to
> assume a
> > different timezone); for interval values, the total number of seconds in
> > the interval"
>
> Hmm, that's not really right either; it appears to imply that the epoch
> calculation is timezone-aware, which it specifically isn't for date and
> timestamp cases.  An example (presuming US DST rules):
>
> regression=# select extract(epoch from date '2020-03-09') - extract(epoch
> from date '2020-03-08');
>  ?column?
> ----------
>     86400
> (1 row)
>
> regression=# select extract(epoch from timestamp '2020-03-09') -
> extract(epoch from timestamp '2020-03-08');
>  ?column?
> ----------
>     86400
> (1 row)
>
> regression=# select extract(epoch from timestamptz '2020-03-09') -
> extract(epoch from timestamptz '2020-03-08');
>  ?column?
> ----------
>     82800
> (1 row)
>
> The last case knows that there was a DST transition in between, the first
> two don't take that into account.  (You could argue that this is more a
> property of the types' input conversion routines than of extract() itself,
> but I think the point is valid anyway.)
>
> Perhaps a better phrasing is "for date and timestamp values, the nominal
> number of seconds since 1970-01-01 00:00:00, without regard to timezone
> or daylight-savings rules".
>
>                         regards, tom lane
>