I am building an application that needs to handle different timezones, including timezones with daylight savings time. I am storing all the dates/times using Postgressql's timestamp with timezone data type, and all are stored in the UTC timezone (I modified the timezone entry in postgresql.conf).
Here is an example table that I have created:
CREATE TABLE category_city
(
category_city_id serial NOT NULL,
appt_start_time timestamp(0) with time zone,
appt_end_time timestamp(0) with time zone,
appt_days_of_week character varying,
CONSTRAINT category_city_pkey PRIMARY KEY (category_city_id )
);
I have been testing some conversions using the 'America\Edmonton' timezone. Currently, this timezone is 6 hours behind UTC. I have a timestamp in this table that has the value 1970-01-01 15:00:00+00.
Now I perform this query:
SELECT appt_start_time at time zone 'America/Edmonton' as start
FROM category_city
This should give me the timestamp 1970-01-01 09:00:00+00, but instead it gives me the timestamp 1970-01-01 08:00:00+00, an offset of -7. which is the correct offset when not in DST, but is obviously not correct right now (DST is in effect).
I must be missing something because I'm sure I'm not the only one that needs to handle different timezones with DST.
The time on my server is correct, this is the output of the date command on Ubuntu:
Wed Apr 30 17:11:59 MDT 2014
Can anyone see something that I am overlooking, or has any experienced something similar and found a way to workaround it? Any help would be appreciated!
At 1970-01-01 09:00:00+00 the time at the timezone 'America/Edmonton' was 1970-01-01 08:00:00 regardless of when you ask. If you want the time at a specific timezone you need to make it explicit:
select '1970-01-01 15:00:00+00' at time zone 'MST' as start;
start
---------------------
1970-01-01 08:00:00
select '1970-01-01 15:00:00+00' at time zone 'MDT' as start;
start
---------------------
1970-01-01 09:00:00
I guess it is easy to see above why that time at the MDT timezone will always be the same.
Related
Let's assume that we have some date, for example 2018-06-25T06:00:00Z. So, in case I do the following I get the same date:
select '2018-06-25T06:00:00Z'::timestamp at time zone 'UTC';
Now, I want to get local time for this date and as I am located in Helsinki I do the following:
select '2018-06-25T06:00:00Z'::timestamp at time zone 'Europe/Helsinki';
As I know that Helsinki is currently +3 hours my expectation is to get 2018-06-25 09:00:00.000000, however, for some reason I get 2018-06-25 03:00:00.000000.
So, why it decided to deduct this 3 hours instead of adding them?
Note, that this select current_setting('timezone') returns UTC.
Your second statement interprets the given timestamp as timestamp in the time zone Europe/Helsinki and converts it to a TIMESTAMP WITH TIMEZONE. psql prints the result of the timestamp according the the SESSION TIMEZONE.
To convert your timestamp from UTC to Europe/Helsinki, you should use a timestamp with time zone:
SELECT '2018-06-25T06:00:00Z'::TIMESTAMPTZ AT TIME ZONE 'Europe/Helsinki';
This will create a timestamp and convert it to Europe/Helsinki. You may change the time zone for output with:
SET SESSION TIMEZONE TO 'Europe/Helsinki';
In my postgresql.conf file, my default timezone is set to 'Europe/London'.
Since the DST will be switched off on 30th October, this means that now (4th October while I am writing) 'Europe/London' should correspond with '+01'.
I have a dummy table with a time column of timestamp with timezone type.
Postgres specs state that:
For timestamp with timezone, the internally stored value is always in UTC (Universal Coordinated Time, traditionally known as Greenwich Mean Time, GMT). An input value that has an explicit time zone specified is converted to UTC using the appropriate offset for that time zone. If no time zone is stated in the input string, then it is assumed to be in the time zone indicated by the system's TimeZone parameter, and is converted to UTC using the offset for the timezone zone.
As for the bold sentence, if I make this insert:
INSERT INTO dummy VALUES ('2016-12-25 12:00:00.000')
I expect it to be interpreted as a 'Europe/London' timezone, thus '2016-12-25 12:00:00.000+01'.
Therefore, when I will retrieve it, I expect to be showed that value (or, at least, the equivalent '2016-12-25 11:00:00.000+00').
Instead, if I do the query
SELECT * FROM dummy
I am returned this:
|time |
|timestamp with time zone|
--------------------------
|2016-12-25 12:00:00+00 |
I can't get the reason of this behaviour. Is the DST management of 'Europe/London' timezone wrong? Am I missing something?
If I switch to any other timezone, it always works as expected.
The timestamp you enter is interpreted in the Europe/London time zone.
On the 25th of December, London will be offset 0 hours from UTC, so noon will be at 12:00 UTC.
Time zone Europe/London is not the same as time zone +01, at least not all the time.
I have a pg database with a column type timestamp with time zone. I inserted the following date:
2016-08-01 00:00:00 GMT
However, in the database, it shows up as:
2016-07-31 20:00:00-04
Does anyone know what might be going on?
Thanks in advance!
Despite the name, TIMESTAMP WITH TIME ZONE doesn't actually store the time zone. It uses the session's time zone to normalize to UTC, and stores UTC. On retrieval it converts back from UTC to the session time zone.
You can change the session time zone by using the SET TIME ZONE command. Preferably, you should use the standard IANA time zone identifiers. For example:
SET TIME ZONE 'Europe/Paris'
or
SET TIME ZONE 'UTC'
Alternatively use the TIMESTAMP [WITHOUT TIME ZONE] type instead, which does no conversions.
I faced with the following issue this morning:
select '2011-12-30 00:30:00'::timestamp without time zone AT TIME ZONE 'EST5EDT';
returns me 2011-12-30 05:30:00+00 witch is wrong.
But next queries below:
select '2011-12-30 00:30:00'::timestamp without time zone AT TIME ZONE 'UTC-5';
select '2011-12-30 00:30:00' AT TIME ZONE 'EST5EDT';
i see right date 2011-12-29 19:30:00
Preventing your question about my local timezone:
SELECT current_setting('TIMEZONE');
current_setting
-----------------
UTC
(1 row)
Do anyone have answer why postgresql converts timestamp without time zone some weird way and instead taking away 5 hours it adds instead?
Key things to understand
timestamp without time zone AT TIME ZONE re-interprets a timestamp as being in that time zone for the purpose of converting it to UTC.
timestamp with time zone AT TIME ZONE converts a timestamptz into a timestamp at the specified timezone.
PostgreSQL uses ISO-8601 timezones, which specify that east of Greenwich is positive ... unless you use a POSIX timezone specifier, in which case it follows POSIX. Insanity ensues.
Why the first one produces an unexpected result
Timestamps and timezones in SQL are horrible. This:
select '2011-12-30 00:30:00'::timestamp without time zone AT TIME ZONE 'EST5EDT';
inteprets the unknown-typed literal '2011-12-30 00:30:00' as timestamp without time zone, which Pg assumes is in the local TimeZone unless told otherwise. When you use AT TIME ZONE, it is (per the spec) re-interpreted as a timestamp with time zone in the time zone EST5EDT then stored as an absolute time in UTC - so it's converted from EST5EDT to UTC, i.e the timezone offset gets subtracted. x - (-5) is x + 5.
This timestamp, adjusted to UTC storage, is then adjusted for your server TimeZone setting for display so that it gets displayed in local time.
If you instead wish to say "I have this timestamp in UTC time, and wish to see what the equivalent local time in EST5EDT is", if you want to be independent of the server TimeZone setting, you need to write something like:
select TIMESTAMP '2011-12-30 00:30:00' AT TIME ZONE 'UTC'
AT TIME ZONE 'EST5EDT';
This says "Given timestamp 2011-12-30 00:30:00, treat it as a timestamp in UTC when converting to timestamptz, then convert that timestamptz to a local time in EST5EDT".
Horrible, isn't it? I want to give a firm talking to whoever decided on the crazy semantics of AT TIME ZONE - it should really be something like timestamp CONVERT FROM TIME ZONE '-5' and timestamptz CONVERT TO TIME ZONE '+5'. Also, timestamp with time zone should actually carry its timezone with it, not be stored in UTC and auto-converted to localtime.
Why the second works (so long as TimeZone = UTC)
Your original "works" version:
select '2011-12-30 00:30:00' AT TIME ZONE 'EST5EDT';
will only be correct if TimeZone is set to UTC, because the text-to-timestamptz cast assumes TimeZone when one isn't specified.
Why the third one works
Two problems cancel each other out.
The other version that appears to work is TimeZone independent, but it only works because two problems cancel themselves out. First, as explained above, timestamp without time zone AT TIME ZONE re-interprets the timestamp as being in that time zone for conversion to a UTC timestamptz; this effectively subtracts the timezone offset.
However, for reasons I beyond my ken, PostgreSQL uses timestamps with the reverse sign to what I'm used to seeing most places. See the documentation:
Another issue to keep in mind is that in POSIX time zone names, positive offsets are used for locations west of Greenwich. Everywhere else, PostgreSQL follows the ISO-8601 convention that positive timezone offsets are east of Greenwich.
This means that EST5EDT is the same as +5, not -5. Which is why it works: because you're subtracting the tz offset not adding it, but you're subtracting a negated offset!
What you'd need to get it correct is instead:
select TIMESTAMP '2011-12-30 00:30:00' AT TIME ZONE 'UTC'
AT TIME ZONE '+5';
I'm having an issue selecting dates properly from Postgres - they are being stored in UTC, but
not converting with the Date() function properly.
Converting the timestamp to a date gives me the wrong date if it's past 4pm PST.
2012-06-21 should be 2012-06-20 in this case.
The starts_at column datatype is timestamp without time zone. Here are my queries:
Without converting to PST timezone:
Select starts_at from schedules where id = 40;
starts_at
---------------------
2012-06-21 01:00:00
Converting gives this:
Select (starts_at at time zone 'pst') from schedules where id = 40;
timezone
------------------------
2012-06-21 02:00:00-07
But neither convert to the correct date in the timezone.
Basically what you want is:
$ select starts_at AT TIME ZONE 'UTC' AT TIME ZONE 'US/Pacific' from schedules where id = 40
I got the solution from this article is below, which is straight GOLD!!! It explains this non-trivial issue very clearly, give it a read if you wish to understand pstgrsql TZ management better.
Expressing PostgreSQL timestamps without zones in local time
Here is what is going on. First you should know that 'PST timezone is 8 hours behind UTC timezone so for instance Jan 1st 2014, 4:30 PM PST (Wed, 01 Jan 2014 16:00:30 -0800) is equivalent to Jan 2nd 2014, 00:30 AM UTC (Thu, 02 Jan 2014 00:00:30 +0000). Any time after 4:00pm in PST slips over to the next day, interpreted as UTC.
Also, as Erwin Brandstetter mentioned above, postresql has two type of timestamps data type, one with a timezone and one without.
If your timestamps include a timezone, then a simple:
$ select starts_at AT TIME ZONE 'US/Pacific' from schedules where id = 40
will work. However if your timestamp is timezoneless, executing the above command will not work, and you must FIRST convert your timezoneless timestamp to a timestamp with a timezone, namely a UTC timezone, and ONLY THEN convert it to your desired 'PST' or 'US/Pacific' (which are the same up to some daylight saving time issues. I think you should be fine with either).
Let me demonstrate with an example where I create a timezoneless timestamp. Let's assume for convenience that our local timezone is indeed 'PST' (if it weren't then it gets a tiny bit more complicated which is unnecessary for the purpose of this explanation).
Say I have:
$ select timestamp '2014-01-2 00:30:00' AS a, timestamp '2014-01-2 00:30:00' AT TIME ZONE 'UTC' AS b, timestamp '2014-01-2 00:30:00' AT TIME ZONE 'UTC' AT TIME ZONE 'PST' AS c, timestamp '2014-01-2 00:30:00' AT TIME ZONE 'PST' AS d
This will yield:
"a"=>"2014-01-02 00:30:00" (This is the timezoneless timestamp)
"b"=>"2014-01-02 00:30:00+00" (This is the UTC TZ timestamp, note that up to a timezone, it is equivalent to the timezoneless one)
"c"=>"2014-01-01 16:30:00" (This is the correct 'PST' TZ conversion of the UTC timezone, if you read the documentation postgresql will not print the actual TZ for this conversion)
"d"=>"2014-01-02 08:30:00+00"
The last timestamp is the reason for all the confusion regarding converting timezoneless timestamp from UTC to 'PST' in postgresql. When we write:
timestamp '2014-01-2 00:30:00' AT TIME ZONE 'PST' AS d
We are taking a timezoneless timestamp and try to convert it to 'PST TZ (we indirectly assume that postgresql will understand that we want it to convert the timestamp from a UTC TZ, but postresql has plans of its own!). In practice, what postgresql does is it takes the timezoneless timestamp ('2014-01-2 00:30:00) and treats it as if it WERE ALREADY a 'PST' TZ timestamp (i.e: 2014-01-2 00:30:00 -0800) and converts that to UTC timezone!!! So it actually pushes it 8 hours ahead instead of back! Thus we get (2014-01-02 08:30:00+00).
Anyway, this last (un-intuitive) behavior is the cause of all confusion. Read the article if you want a more thorough explanation, I actually got results which are a bit different then their on this last part, but the general idea is the same.
I don't see the exact type of starts_at in your question. You really should include this information, it is the key to the solution. I'll have to guess.
PostgreSQL always stores UTC time for the type timestamp with time zone internally. Input and output (display) are adjusted to the current timezone setting or to the given time zone. The effect of AT TIME ZONE also changes with the underlying data type. See:
Ignoring time zones altogether in Rails and PostgreSQL
If you extract a date from type timestamp [without time zone], you get the date for the current time zone. The day in the output will be the same as in the display of the timestamp value.
If you extract a date from type timestamp with time zone (timestamptz for short), the time zone offset is "applied" first. You still get the date for the current time zone, which agrees with the display of the timestamp. The same point in time translates to the next day in parts of Europe, when it is past 4 p.m. in California for instance. To get the date for a certain time zone, apply AT TIME ZONE first.
Therefore, what you describe at the top of the question contradicts your example.
Given that starts_at is a timestamp [without time zone] and the time on your server is set to the local time. Test with:
SELECT now();
Does it display the same time as a clock on your wall? If yes (and the db server is running with correct time), the timezone setting of your current session agrees with your local time zone. If no, you may want to visit the setting of timezone in your postgresql.conf or your client for the session. Details in the manual.
Be aware that the timezone offset used the opposite sign of what's displayed in timestamp literals. See:
Peculiar time zone handling in a Postgres database
To get your local date from starts_at just
SELECT starts_at::date
Tantamount to:
SELECT date(starts_at)
BTW, your local time is at UTC-7 right now, not UTC-8, because daylight savings time is in effect (not among the brighter ideas of the human race).
Pacific Standard TIME (PST) is normally 8 hours "earlier" (bigger timestamp value) than UTC (Universal Time Zone), but during daylight saving periods (like now) it can be 7 hours. That's why timestamptz is displayed as 2012-06-21 02:00:00-07 in your example. The construct AT TIME ZONE 'PST' takes daylight saving time into account. These two expressions yield different results (one in winter, one in summer) and may result in different dates when cast:
SELECT '2012-06-21 01:00:00'::timestamp AT TIME ZONE 'PST'
, '2012-12-21 01:00:00'::timestamp AT TIME ZONE 'PST'
I know this is an old one but You may want to consider using AT TIME ZONE "US/Pacific" when casting to avoid any PST/PDT issues. So
SELECT starts_at::TIMESTAMPTZ AT TIME ZONE "US/Pacific"
FROM schedules
WHERE ID = '40';