All the above are the exact same point in time merely stated as
relevant to each location. Note that given a timestamp with time zone
and a zone, PostgreSQL returns a timestamp without time zone (you know
the zone since you specified it).
Yes, I know the zone. But I don't know the offset from UTC.
Example:
template1=> set timezone to 'UTC';
SET
template1=> select ('2011-10-30 01:00:00'::timestamptz) at time zone
'Europe/Budapest';
timezone
---------------------
2011-10-30 02:00:00 -- Is it winter or summer time?
(1 row)
template1=> select ('2011-10-30 00:00:00'::timestamptz) at time zone
'Europe/Budapest';
timezone
---------------------
2011-10-30 02:00:00 -- Is it winter or summer time? What is the
offset from UTC here? Can you tell me when it was in UTC?
(1 row)
template1=>
What is more:
template1=> select (('2011-10-30 00:00:00'::timestamptz) at time zone
'Europe/Budapest') is distinct from (('2011-10-30
01:00:00'::timestamptz) at time zone 'Europe/Budapest');
?column?
----------
f
(1 row)
template1=>
Yeah, we know what time zone it is in, but we don't know when it was,
thanks a lot. :-( It would be unambiguous to store the UTC offset along
with the value. But it is not how it was implemented.
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