I'm using postgreSQL 10.1.
I have datetimes that have been created in UTC, in timestamp without time zone columns.
I need to return these as the correct UK local time for each (GMT or BST).
I've been trying AT TIME ZONE, and time zones BST/GMT seem fixed to their own offsets, regardless of when in the year the timestamp occured.
AT TIME ZONE 'UTC' AT TIME ZONE 'Europe/London' seems to give me what i want, showing a UTC timestamp as GMT or BST correctly (in the small sample i've tried it on), though i can't find this time zone properly documented regarding its daylight savings rules.
Can anyone point me in the right direction, or to a reliable alternative?
Your solution is correct:
AT TIME ZONE 'UTC' AT TIME ZONE 'Europe/London'
The first assigns UTC to the timestamp, the second converts from UTC to the Europe/London time zone.
Postgres documentation states:
... PostgreSQL uses the widely-used IANA (Olson) time zone database for information about historical time zone rules. ...
It also supports other forms of time zones, referenced in the same documentation, but Europe/London is an IANA time zone. You can see it in the list of IANA time zones here. You can also look at the details of how this time zone is defined by examining the TZDB sources here, or view a more human-readable history of time changes in London here.
Related
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 have column 'event_date_time' in my 'events' table with type 'timestamp with timezone'. My python flask application is saving date like '2014-08-30 02:17:02+00:00' but postgres automatically converts it to '2014-08-30 07:17:02+05'. It converts the timestamp to my local timezone i-e Pakistan. I want to save it without converting.
I have tried
set timezone='UTC'
and it does change timezone to 'UTC' but pgadmin3 is still saving the converted time.
I am using MAC OS and Postgresql 9.3.
The reason pgadmin is displaying hours +5 is because your system timezone is set to this.
When you save a "timestamp with time zone" value at GMT + or - any value, the system offsets whatever timezone your input was to GMT (or UTC), so that when you go to retrieve it, you can specify the timezone you want it displayed in.
For example let's establish a current time for say... New York.
select now()::timestamp with time zone at time zone 'America/New_York';
At the time of asking it returned '2014-08-23 08:50:57.136817'. 8:50 Saturday morning, or 8:51 if you're being pedantic.
Now if we take that same time and display it in GMT we will see a different result:
select '2014-08-23 08:50:57.136817 America/New_York'::timestamp with time zone at time zone 'GMT';
Now have a new time of '2014-08-23 12:50:57.136817'... 5 hours into the "future"!
Finally let's get the original timestamp and display it in what I believe is the Pakistan time zone (PKT) and see what it shows
select '2014-08-23 08:50:57.136817 America/New_York'::timestamp with time zone at time zone 'PKT';
The result? '2014-08-23 17:50:57.136817' further into the future still!
Again I must stress the reason it can do this is because it is always converting the input time offset to UTC or GMT. Postgres processes all of its "timestamp with time zone" data types in this way. It is designed to avoid time zone problems such as daylight savings and so on.
Your issue appears to be that python is inserting the time at an offset of +00, and if this was supposed to be a local time then you will be 5 hours off as far as postgres is concerned. Without knowing exactly what queries python is making, I would assume you may want to look at that to make sure it is giving you the correct time, presumably set timezone='PKT' should be a fix. Either way, when you are viewing timestamp with time zone using a browser such as pgadmin, the timestamp is being converted to your local timezone and this is why you see +5.
Alternatively if you do wish to see those times at +00 then you must specify that you want this in your SELECT queries.
On 9.3.3, if one runs:
select
EXTRACT(TIMEZONE FROM timestamp with time zone '1911-03-01 00:00 -8:00:00'),
EXTRACT(TIMEZONE FROM timestamp with time zone '1911-05-15 00:00 -8:00:00'),
EXTRACT(TIMEZONE FROM timestamp with time zone '1917-03-01 00:00 -8:00:00'),
EXTRACT(TIMEZONE FROM timestamp with time zone '1917-05-15 00:00 -8:00:00'),
EXTRACT(TIMEZONE FROM timestamp with time zone '1967-03-01 00:00 -8:00:00'),
EXTRACT(TIMEZONE FROM timestamp with time zone '1967-05-15 00:00 -8:00:00'),
EXTRACT(TIMEZONE FROM timestamp with time zone '1968-03-01 00:00 -8:00:00'),
EXTRACT(TIMEZONE FROM timestamp with time zone '1968-05-15 00:00 -8:00:00');
One gets the following results:
0;0;
0;3600;
0;3600;
3600;3600
(The first time is the founding day of Las Vegas, the next few are some I used to debug the issue)
It seems there is no offset around 1911, an offset between 1911 and 1967 during summer but not winter and then always has one from 1968 onwards. This seems a little weird. Does anyone have any idea what is going on with the offsets here and whether this is expected behaviour or if there is something in my linux's setup that I could possibly change?
Time zones change for all sorts of reasons.
Daylight savings rules change.
Sometimes timezone offsets change, too, if nations redefine their time zone for political reasons.
The canonical time zone information database is the tz or "zoneinfo" database, which used to be called the Olsen database. The zoneinfo DB is on the IANA site. There are a variety of programs to dump human readable versions of the DB.
You can use timestamp without time zone if you wish to store a particular moment in wall-clock time, without concern for time zone.
timestamp with time zone is sensitive to the system TimeZone setting on input and output, and is stored in UTC time as absolute seconds. So it's converted for input and output. If you want different conversions or to override the conversion you can use the AT TIME ZONE operator.
The rules for your time zone are established by law, and the law changes.
If you want to store the INSTANT at which the (local) clocks in Las Vegas marked 00:00:00 in the day in which the city was founded, and assuming that Las Vegas was using an offset of -8 hours, then you should store 1911-03-01 00:00 -8:00:00::timezonetx in a timezonetx field. Be aware, however, that what is really stored is only the "universal instant", when you read it you cannot know to which "local time at Las Vegas" it corresponds (unless you explicityl convert it, after reading it, to a timezone).
Depends on your timezone, really. In 1966, DST was first implemented nationally in the US to take place in 1967. States needed to pass laws to end it, meaning that in many areas, 1967 is the only year which used DST. This can lead to some very interesting glitches, where every date except those in 1967/68 behaves normally.
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';