How do I tell postgres a timestamp within a column is UTC?

问题

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

回答1:

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.

回答2:

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:

1. 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.

2. 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.

来源：https://stackoverflow.com/questions/27299234/how-do-i-tell-postgres-a-timestamp-within-a-column-is-utc