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

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