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
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