Migrate datetime w. timezone in PostgreSQL to UTC timezone to use Django 1.4

Question

Do I need to convert all existing datetime in Postgres from their current timezone to UTC or will Django 1.4 timezone understand when reading something that is not UTC (but

Answer 1

I am storing all datetime w. timezone info but this happens to be set to US/Eastern timezone (yes, I know should have been UTC) in Postgres.

There are a couple of misconceptions here.

The data type is called timestamp in PostgreSQL. There is no type called "datetime".
timestamp is short for timestamp without time zone.
timestamptz is short for timestamp with time zone.

As the manual informs:

timestamp values are stored as seconds before or after midnight 2000-01-01.

Similar to Posix time, which start 30 years earlier at the Unix epoch 1970-01-01 00:00 UTC. For timestamp, the local 2000-01-01 00:00 is assumed. For timestamptz the reference is 2000-01-01 00:00 UTC and values get adjusted for the time zone offset on input and output.

timestamp with time zone is just another way to input and represent a unique point in time. You cannot "set" a timestamp (with or without time zone) to any other time zone than UTC internally. The time zone offset itself is not saved at all. It is only used to adjust the value to UTC.

The representation of the timestamp value will take the current time zone setting into account

• to display the value accordingly (output)
• to interpret a timestamp without time zone (input).

The good news: your migration should just work out of the box - as long as you don't screw it up actively.

I have written a detailed explanation of how Postgres timestamps work with examples and links in this related answer.

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Example queries

Try the following statements (one block at a time). And try it with your column, too:

SHOW timezone;

SELECT '2011-05-24 11:17:11.533479-05'::timestamptz(0);
SELECT '2011-05-24 11:17:11-05'::timestamptz;

SET timezone='UTC';
SELECT '2011-05-24 11:17-05'::timestamptz;

SELECT '2011-05-24 11:17-05'::timestamptz AT TIME ZONE 'UTC';

SELECT '2011-05-24 11:17-05'::timestamptz AT TIME ZONE 'UTC' AT TIME ZONE 'UTC';

RESET timezone;