For example, certain time values might auto change when the system time zone changes. Will a timestamp without time zone value that's already stored in the database auto change for any reason at all?
Neither timestamp (timestamp without time zone) nor timestamptz (timestamp with time zone) values will ever change by themselves. Only the display of timestamptz adapts to the current timezone setting.
Or you can display date and time for a given time zone with the AT TIME ZONE construct. And / or you can format it any way you like with various formatting functions.
The actual setting in your session is relevant for display, not the system time zone. Related:
Ignoring time zones altogether in Rails and PostgreSQL
Time zone storage in data type "timestamp with time zone"
I'm gonna to save date and time in my PostgreSQL database and fetch and show it to the user in an appropriate format.
Suppose that application users are located in Iran and use Jalali date/time system. In the first half of year, Iran time is UTC+04:30, but in the rest of year, it is UTC+03:30. As a matter of fact daylight saving time is used in Iran.
IMPORTANT: Sometimes we make a decision and change database server location from Iran to Europe or elsewhere.
Now I have some questions:
What data type is more convenient to save date with time? TIMESTAMP (TIMESTAMP WITHOUT TIMEZONE) or TIMESTAMPZ (TIMESTAMP WITH TIME ZONE)?
What data type is more convenient to save time only? TIME or TIME WITH TIME ZONE?
What data type is more convenient to save date only? DATE (Gregorian) String (Jalali) or a custom data type?
How can I set TIMEZONE for our database once? I wanna set TIMEZONE for database one time and after that all queries over TIMESTAMPZ columns will be saved and fetched within that TIMEZONE and also it considered daylight saving time ?
What SQL query is the best when I wan to save current date and time ?
a- INSERT INTO test(d) VALUES(now());
b- INSERT INTO test(d) VALUES(now() at time zone 'utc');
c- INSERT INTO test(d) VALUES(now() at time zone 'Asia/Tehran');
d- INSERT INTO test(d) VALUES(current_timestamp);
e- INSERT INTO test(d) VALUES(now() at time zone 'utc');
f- INSERT INTO test(d) VALUES(now() at time zone 'Asia/Tehran');
Is number 5-c and 5-f considered daylight saving times when save it or not?
Is number 5-a and 5-d saved in 'Asia/Tehran' when database has 'Asia/Tehran' time zone ?
When I wan to query from database, what options is the best in my situation ?
a- SELECT d FROM test;
b- SELECT d at time zone 'utc' FROM test;
c- SELECT d at time zone 'Asia/Tehran' FROM test;
Is number 7-c considered daylight saving times?
Is number 7-a considered 'Asia/Tehran' time zone and daylight saving time when database has 'Asia/Tehran' time zone?
If I'd better use the timestamp to save date/time, then I have to add daylight saving time to it and convert it to Jalali date time and show it to the user, and vice versa.
https://www.postgresql.org/docs/current/static/datatype-datetime.html
For timestamp with time zone, 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.
When a timestamp with time zone value is output, it is always
converted from UTC to the current timezone zone, and displayed as
local time in that zone. To see the time in another time zone, either
change timezone or use the AT TIME ZONE construct
timestamptz
timestamptz and select date only
timestamptz and select time only
database saves tz aware time stamps in UTC, no matter your locale or settings. stamps are always converted to UTC adjusting it by TimeZone parameter. Same deconvert happens on displaying data every time. TimeZone for server is only default value, used if client does not specify any. database TimeZone overrides the one in postgresql.conf for specified database, but still, client settings will override the database ones.
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.
We have an application that fetches data from a source and that source present the data with a timestamp in UTC. When our application saves that data to Postgres, it stores that timestamp in a timestamp column without time zone. The default on postgres in our shop is set to our local time, Mountain Time. So that means, I think, that postgres assumes that timestamp is mountain time. How can I query that column so that my result set thinks it's UTC and not the local time zone?
More cleary stated, I need to perform some offsets on that timestamp (moving it to, say EST) and so the math of doing that is different if the resultset thinks it's UTC than my local time
The Answer by Kouber Saparev is mostly correct, though incorrect about storing a time zone.
Wrong data type in Postgres
a timestamp in UTC. When our application saves that data to Postgres, it stores that timestamp in a timestamp column without time zone.
As noted in his Answer, you are using the wrong data type in your Postgres database. When tracking moments, you must use a column of type TIMESTAMP WITH TIME ZONE. When supplying an input during an insert or update, any accompanying info about time zone or offset-from-UTC is used to adjust into UTC. The accompanying zone/offset is then discarded. If you need to remember the original zone/offset, you will need to define a second column and store that info there yourself.
The other type in Postgres, and the SQL standard, is TIMESTAMP WITHOUT TIME ZONE. This type purposely lacks any concept of time zone or offset-from-UTC. So this type cannot represent moments, cannot store points on the timeline. It stores values that represent potential moments along a range of about 26-27 hours, the range of various time zones around the globe. Use this type only when you mean a date with time-of-day everywhere or anywhere, but not specifically somewhere. Also used when you mean appointments far enough out in the future that we run the risk of politicians changing the offset used in any of the time zones we care about.
Always specify time zone
default on postgres in our shop is set to our local time, Mountain Time
Never depend on the current default time zone of your host OS, the database server, or your tools such as the Java Virtual Machine. Always specify the desired/expected time zone in your code.
Tip: Generally best to work in UTC for data storage, data exchange, and most of your business logic. Adjust from UTC to a time zone only for presentation to the user or where business rules require.
As explained above, Postgres always stores date-time values either in UTC or with no zone/offset at all. Beware: Tools used between you and Postgres may apply a time zone to the UTC value retrieved from the database. While well-intentioned, this anti-feature creates the illusion that the time zone was stored when in fact only UTC was stored in TIMESTAMP WITH TIME ZONE or no zone/offset at all in TIMESTAMP WITHOUT TIME ZONE.
Be aware that any zone information accompanying input to a column of TIMESTAMP WITHOUT TIME ZONE is simply ignored, the date and time-of-day taken as-is and stored.
I need to perform some offsets on that timestamp (moving it to, say EST)
Generally best to use your database just for storage, query, and retrieval of data. For massaging the data like adjusting time zone, do such work in your application. For example, in Java use the industry-leading java.time classes, in .NET the Noda Time project (a port of the predecessor of java.time, the Joda-Time project).
Example code in Java using JDBC 4.2 or later.
LocalDateTime
For a value in a column of TIMESTAMP WITHOUT TIME ZONE we use the corresponding type in Java, LocalDateTime, lacking any concept of time zone or offset-from-UTC.
LocalDateTime ldt = myResultSet.getObject( … , LocalDateTime.class ) ; // Retrieve value from database.
String output = ldt.toString() ; // Generate text representing this date-with-time value in standard ISO 8601 format.
2018-01-23T01:23:45.123
If you know for certain that this date and time was meant for UTC but was incorrectly stored without any zone/offset info, you can apply a zone or offset to repair the damage.
OffsetDateTime odt = ldt.atOffset( ZoneOffset.UTC ); // Apply an offset-from-UTC to a `LocalDateTime` lacking such information. Determines a moment.
OffsetDateTime
For a value in a column of TIMESTAMP WITH TIME ZONE we use the corresponding type in Java, OffsetDateTime (or Instant), representing a moment in UTC.
OffsetDateTime odt = myResultSet.getObject( … , OffsetDateTime.class ) ; // Retrieve value from database.
String output = odt.toString() ; // Generate text representing this date-with-time value in standard ISO 8601 format. A `Z` on the end indicates UTC, pronounced “Zulu”.
2018-01-23T01:23:45.123Z
ZonedDateTime
To see that OffsetDateTime value set in UTC through the lens of the wall-clock time used by the people of regions within the mid-west of North America, specify a time zone such as America/Edmonton or America/Denver.
Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 2-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).
ZoneId z = ZoneId.of( "America/Denver" ) ;
ZonedDateTime zdt = odt.atZoneSameInstant( z ) ;
See this code run live at IdeOne.com. We see the same moment but with a different wall-clock time.
2018-01-22T18:23:45.123-07:00[America/Denver]
Beware of tools & middleware injecting a time zone
Unfortunately, many tools and middleware will volunteer to apply some default time zone to a moment retrieved from the database. While well-intentioned, this creates the illusion of the zone having been a part of the stored data when in fact the time zone was added after storage, upon retrieval. This anti-feature creates much confusion. I wish all the tools were clear and truthful by reporting the moment in UTC, as it was stored.
If you use Java, with JDBC 4.2 and later, you can exchange java.time (JSR 310) (tutorial) objects with the database and avoid this time zone injection.
There are two data types handling timestamps in PostgreSQL - timestamp, and timestamptz (timestamp with time zone). The latter stores the time zone along with the timestamp itself.
If you are using just a timestamp without time zone, then there is no way for the result set to think whether the timestamp is UTC or not. It is just a timestamp. It is up to the client application to interpret it and give it some time zone meaning.
On the contrary, if you use timestamptz, then PostgreSQL knows the time zone of that timestamp, and then it can calculate time zone offsets properly for you.
db=# select now();
now
-------------------------------
2014-12-04 19:27:06.044703+02
(1 row)
db=# select timezone('est', now());
timezone
----------------------------
2014-12-04 12:27:06.044703
(1 row)
So, back on the problem posed. You need to make sure that first the data is imported properly and then - when needed, it is returned and displayed properly to the end user. You have two options:
Continue using timestamp
In that case both the writing app and the reading app need to know that all the timestamps in the database are UTC and calculate offsets accordingly.
Switch to timestamptz
Then the only thing that the apps need to know is their own time zone, they just have to declare it after connecting to PostgreSQL and leave the rest to the database.
For example, let's connect as a writing app and declare our time zone as UTC.
db=# create table x (data timestamptz);
CREATE TABLE
db=# set timezone='utc';
SET
db=# insert into x values (now());
INSERT 0 1
db=# select * from x;
data
-------------------------------
2014-12-04 20:02:08.692329+00
(1 row)
Now, let's say a reading app connects and is in the EST time zone.
db=# set timezone='est';
SET
db=# select * from x;
data
-------------------------------
2014-12-04 15:02:08.692329-05
(1 row)
Changing the client time zone setting changes the way all the timestamps are returned, but that's the case only if you use timestamptz - timestamp with time zone. If you cannot switch to this data type, then the application will have to take care of all this magic.
I need to convert local time to UTC using a function. The inputs I have is the local time, timezone of the local time (e.g. 'Pacific/Auckland'). What I need to get from the procedure is the UTC time for the local time based on given timezone.
Can someone please help?
I am using version 8.3
This is covered in the manual, but it's not always obvious how to actually work with dates/times. The SQL spec is a bit bizarre.
In the case of your question it isn't clear whether you want to store the time in UTC but display it in the server's local time (TimeZone), or whether you wan to ignore TimeZone and always display it as UTC. I'll assume the latter.
For timestamps you'd just use AT TIME ZONE twice, like:
SELECT TIMESTAMP '2013-08-13 00:00:00' AT TIME ZONE 'Australia/Sydney' AT TIME ZONE 'UTC';
You need to use AT TIME ZONE twice. Once to convert the input timestamp to timestamptz according to the argument timezone, then another to convert that to a timestamp at UTC.
Unfortunately because of the (IMO insane) way the SQL spec defines AT TIME ZONE for TIME, you can't do the same thing for TIME. You'll have to manipulate the TimeZone variable instead:
SET TimeZone = 'UTC';
SELECT TIME '07:00' AT TIME ZONE 'Australia/Sydney';
This still leaves you with a timetz not a time. So its display value changes with the timezone setting.
You can use select now() at time zone 'UTC' for postgresql.