Footguns with Postgres "at time zone 'UTC'"

The text says: "Comparing a timestamp and timestamptz will always result in false."

That is wrong. Postgres changes the timestamp to timestamptz very quietly in the background. Wether it uses the time zone of the session for this. If it is true or false depends on the TimeZone setting. This is more bad than "always false". In production with UTC it works. On a laptop of a California developer it does not work.

Just tested:

  SET TIME ZONE 'America/Los_Angeles';
  SELECT '2026-03-01 00:00:00'::timestamp = '2026-03-01 00:00:00+00'::timestamptz; /* f   */

  SET TIME ZONE 'UTC';
  SELECT '2026-03-01 00:00:00'::timestamp = '2026-03-01 00:00:00+00'::timestamptz; /* t  */

If you can, use Postgres 16. The double AT TIME ZONE 'UTC' thing from the text is not needed there. There you have date_add with a time zone as the third argument:

date_add(b.month_start, interval '1 month', 'UTC')

This adds the month in UTC and it stays a timestamptz.

The SQL standard is unfortunately really horrible when it comes to handling of time. The type `timestamp` is not a timestamp at all because it doesn't encode a unique point in time, it just stores a date and a time which has to be interpreted relative to a timezone. It should be called "datetime".

Moving a Java Instant back and forth between a database is also a surprisingly difficult task to do right, and it doesn't help that JDBC is just handling it completely wrong if you use its setTimestamp/getTimestamp methods. Not because it is a bad design with footguns, but because the implementation is just plain wrong and will corrupt your data if you deal with instants whose calendar date is far enough in the past due to it using the legacy date/time API which switches to the Gregorian calendar for dates in the past.

The name `timestamp with time zone` is also misleading because it doesn't actually store a time zone, it stores the number of seconds since epoch like a java.time.Instant (although at a different resolution). The "with time zone" part just refers to the textual format you denote the values in which includes the time zone after the date/time part to uniquely identify a timestamp, but the time zone is thrown away and not stored after the value has been parsed. This is different from e.g. `ZonedDateTime` in Java which will actually store the offset and therefore corresponds to a pair of (Instant, TimeZone).

I don't rely on time calculations at the database level. I handle them in the application based on UTC time stored in the database and the user's time zone. This shifts the problem to the application code, where I can control it precisely and make conscious decisions about how to handle specific business requirements, such as when a day ends or how to deal with events across different time zones. It also makes it possible to properly test all cases with unit tests.

The more I read about pgsql, the more I wonder why I would want to switch to it...

Granted, mariadb has weird time things too... but thats the tip of the iceberg between autoincrement with vacuum, vacuum in general, and that thread from the other day about bad migrations/version upgrades...

> Adding a month with + INTERVAL '1 months' is timezone-dependent. […]

Adding months isn’t well-defined anyway, even when using date, for days of month > 28. I think it’s a mistake that systems generically allow such a computation (as opposed to application code implementing domain-specific business rules).

I think most of the confusuion comes from "AT TIME ZONE" being the syntax for both the conversion from _and_ to timezone'd timestamps. Maybe it would have been more intuitive had the two operations gotten different wordings, e.g.:

timestamp to timestamptz: AS ZONED AT TIME ZONE ...

timestamptz to timestamp: AS LOCAL AT TIME ZONE ...

Postgres 'at time zone' is confusing, but it's internally consistent. Once you understand what it's doing, you can work with it and it will reliably behave the way it's designed to.

https://oneuptime.com/blog/post/2026-01-25-postgresql-timezo...

    -- TIMESTAMPTZ AT TIME ZONE 'X' -> returns TIMESTAMP in timezone X
    -- TIMESTAMP AT TIME ZONE 'X' -> returns TIMESTAMPTZ treating input as timezone X

Makes me wonder what kind of footgun we are talking about here — is it classic, smoking, standard or vanilla?

good lord, now we have LLM-speak "footguns" in article titles. what have we come to.

what about the load bearing real unlocks of the footguns?!??!?

https://i.programmerhumor.io/2026/09/09db9203bc618d8704855e1...

Git gud. Don’t mix derived data from recorded data. Record data with the information available, then normalise for processing.

Don’t be a monkey.