PostgreSQL date() with timezone

Question

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.

Answer 1

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.

Answer 2

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'