Hi hackers,
Due to DST and also changes in local laws, there could be gaps in
local time [1]. For instance, 1 second after "2011-03-27 01:59:59 MSK"
goes "2011-03-27 03:00:00 MSK":
```
select (timestamptz '2011-03-27 01:59:59 MSK') at time zone 'MSK';
timezone
---------------------
2011-03-27 01:59:59
(1 row)
select ((timestamptz '2011-03-27 01:59:59 MSK') + interval '1 second')
at time zone 'MSK';
timezone
---------------------
2011-03-27 03:00:00
(1 row)
```
This makes '2011-03-27 02:00:00 MSK' an impossible timestamptz. I was
curious how `timezone(zone, timestamp)` aka `timestamp at time zone`
handles such dates and discovered that it seems to round impossible
dates to the nearest possible one:
```
set time zone 'Europe/Moscow';
select (timestamp '2011-03-27 01:00:00') at time zone 'MSK';
timezone
------------------------
2011-03-27 01:00:00+03
(1 row)
select (timestamp '2011-03-27 02:00:00') at time zone 'MSK';
timezone
------------------------
2011-03-27 01:00:00+03
(1 row)
```
I don't know what the SQL standard says about it, but personally, I
find this behavior very convenient. Although it doesn't seem to be
documented [2].
So I have two questions:
1. Should this behavior be documented in the 9.9.4. AT TIME ZONE
section or maybe it's documented elsewhere and I just missed it?
2. Is it possible to detect an impossible timestamptz's for users who
wants stricter semantics? If there is a way I think it's worth
documenting as well.
[1]: https://en.wikipedia.org/wiki/Moscow_Time#Past_usage
[2]: https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-ZONECONVERT
--
Best regards,
Aleksander Alekseev
