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heurekamala 2 hours ago [-]
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:
This adds the month in UTC and it stays a timestamptz.
whizzter 32 minutes ago [-]
Automatic conversion using "global" state between "local"/"human" time (5pm where I am now) and points in time (ie with timezone) is one of the biggest sins imho for many libraries/db's/languages. Ran into it a bunch of times using C# as well.
ulrikrasmussen 3 hours ago [-]
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).
masklinn 3 hours ago [-]
A ZonedDateTime is not an instant and a timezone, for the same reason that you can’t unambiguously round trip between arbitrary timezones and UTC:
- zoned datetimes carry ambiguities as to their actual location on the timeline (because they can repeat, or not exist at all)
- future zoned datetime carry outright uncertainty as to their actual location on the timeline (as the zone’s offset can be updated at any point and any number of times until the event has elapsed)
rawling 12 minutes ago [-]
Has anyone proposed versioning timezones? Or is this such an edge case it would be overkill? (Either specify your future instant in UTC if you mean to stick to that, or specify it in a timezone and accept that it could change before it happens, or if you need something else get it in a contract and don't trust the computer!)
ulrikrasmussen 2 hours ago [-]
Ah, yes, you are right! Forgot about those two details
officialchicken 3 hours ago [-]
SET TIME ZONE 'UTC'; // or GMT, PST, etc.
crote 1 hours ago [-]
Except you shouldn't do the GMT/PST part, because it can lead to people falsely believing that Postgres can handle time zones, leading to silent data corruption when the time zone definition changes - such as due to abolishing DST.
Postgres can only store 1) unzoned local time, or 2) UTC, with optional conversion on read/write.
This is why you store two dates: what the user sent and what you need for calculations. What the user sent becomes your oracle and what you show the user (date time + timezone) and your derived utc to do datetime arithmetic
ekejjedjndbd 10 minutes ago [-]
If the UTC is derived why store it (unless just as a performance concern e.g. materialised view type of thing)
beybol 2 hours ago [-]
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.
wodenokoto 2 hours ago [-]
So how are you solving the example in the article where they join on timestamps? Read both tables from the DB into the application?
beybol 1 hours ago [-]
I read raw data (UTC timestamps) from the db and timezone for user profile and calculate it on the fly.
threatofrain 2 hours ago [-]
We can think of the split between DB and application in terms of DX or code, or we can think of it in terms of colocation of data and compute. The latter case will be compelling sometimes.
ahoka 2 hours ago [-]
I've seen a product where they used UTC to store opening hours. They had to rewrite all dates via a script twice a year.
beybol 55 minutes ago [-]
I don't know the exact use case, but it sounds weird. If UTC is the single source of truth, then when you display the data and want to change some business assumptions, you only need to change the application logic. The underlying data remains unchanged.
One scenario I can imagine is when you want to store the results of business calculations in the database. In that case, once the algorithm changes, you may also need to update the stored data. This may be necessary for performance reasons.
In all other cases, calculating the result on the fly solves the problem and does not require a database update when the business rules change.
GJim 2 hours ago [-]
Why would the dates need rewriting?
sokoloff 2 hours ago [-]
I suspect they meant more generally “the date&time field”, but a store in Boston that opens at 7:30 AM and closes at 7:30 PM Eastern time (ET) closes on different UTC date than it opens when ET is EST but opens and closes on the same date when ET is EDT, so it’s plausible that the dates actually needed to be updated.
GJim 2 hours ago [-]
This is the only way. To do otherwise smacks of poor programming practice and is very often a sign the coder doesn't habitually consider the world outside their own timezone.
layer8 3 hours ago [-]
> 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).
Felk 4 hours ago [-]
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 ...
nubinetwork 2 hours ago [-]
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...
codesnik 2 hours ago [-]
postgres is sane and consistent. Some things could seem weird, but those usually track to SQL standards, which are weird and dated, but at least consistency is still there. On the other hand, I never can be sure where mysql/mariadb decided to improve my life.
nubinetwork 2 hours ago [-]
Oh believe me, I dislike mariadb for other reasons, like the fact that it sucks at picking indexes for queries.
ForHackernews 3 hours ago [-]
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.
-- TIMESTAMPTZ AT TIME ZONE 'X' -> returns TIMESTAMP in timezone X
-- TIMESTAMP AT TIME ZONE 'X' -> returns TIMESTAMPTZ treating input as timezone X
globular-toast 2 hours ago [-]
> timestamp doesn't contain the timezone information. Technically, it doesn't represent a time in the real world.
Unfortunately this is just as misleading.
It's correct that `timestamp` doesn't contain timezone information, but *neither does `timestamptz`. All `timestamptz` is is UNIX-style Epoch timestamp, that is an absolute point in time in UTC. It says nothing about the timezone because there are no time zones in that context.
The confusion arises because when you insert into a field you can insert "19:00 on the 29th March 2025 in London", but this is simply converted to UTC at insert time and the timezone is lost.
If anything, `timestamp` does represent a time in the "real world" and `timestamptz` represents a time in the universe.
If time zones are important to you, you need to record the time zone in a separate field. That is all. Where you store this depends on context, of course. You could store the user's current time zone in a profile and render all times in that time zone (psql and other clients do this automatically based on the OS time zone btw). Or you could store the time zone with each record if that makes sense, ie. you want to know what the wall clock time was for the user at the time the record was taken.
ScanMyTerms 7 minutes ago [-]
[flagged]
paulddraper 3 minutes ago [-]
TLDR Always use timestamptz (timestamp with time zone).
Only use timestamp (timestamp without time zone) if you really need local/plain datetime.
(Ideally, timestamptz would be called timestamp/instant, and timestamp would be called datetime.)
walrus01 4 hours ago [-]
[flagged]
FooBarWidget 4 hours ago [-]
? Footguns as a synonym for "caveat" or "gotcha" were in use long before LLMs. I hate Claude speak too but there's no need to knee-jerk blame everything on LLMs.
walrus01 4 hours ago [-]
While that is true (the LLMs surely had to learn it from pre-existing content), I think it's been so thoroughly overused and turned into a cliche that the term has been ruined for the foreseeable future.
wongarsu 3 hours ago [-]
Looking forward to the blog post "Term footgun considered harmful"
cloudie78 2 hours ago [-]
[flagged]
cowboylowrez 46 minutes ago [-]
the greater fun to be had is doing integration between two systems with differing ideas of how to store time. My experience was with big erp that stored utc and little country store hack that stored local time. I sorta learned why I like utc, but there are plenty of interesting gotchas with time thats for sure!
estetlinus 3 hours ago [-]
Makes me wonder what kind of footgun we are talking about here — is it classic, smoking, standard or vanilla?
officialchicken 3 hours ago [-]
Rookie pseudo-footgun - assumed skill exceeds capability error - usually accompanied with blameshifting pronunciation as demonstrated here. Anyone familiar with UTC/tz from another platform (e.g. excel) would be mostly immune or able to quickly identify and remedy.
TeMPOraL 2 hours ago [-]
People who handle this footgun properly tend to come from two groups:
- Those with safety training for this class of foot-pointed firearms, who know how dumb shit looks like and that they should not do it;
- Those with officer training who are able to recognize the higher-level categories, and take principled approach to safety - e.g. recognizing that "date", "timestamp, "duration", "time of day", "time of week", etc. are different concepts and should not be mixed.
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:
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.
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).
- zoned datetimes carry ambiguities as to their actual location on the timeline (because they can repeat, or not exist at all)
- future zoned datetime carry outright uncertainty as to their actual location on the timeline (as the zone’s offset can be updated at any point and any number of times until the event has elapsed)
Postgres can only store 1) unzoned local time, or 2) UTC, with optional conversion on read/write.
One scenario I can imagine is when you want to store the results of business calculations in the database. In that case, once the algorithm changes, you may also need to update the stored data. This may be necessary for performance reasons.
In all other cases, calculating the result on the fly solves the problem and does not require a database update when the business rules change.
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).
timestamp to timestamptz: AS ZONED AT TIME ZONE ...
timestamptz to timestamp: AS LOCAL AT TIME ZONE ...
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...
https://oneuptime.com/blog/post/2026-01-25-postgresql-timezo...
Unfortunately this is just as misleading.
It's correct that `timestamp` doesn't contain timezone information, but *neither does `timestamptz`. All `timestamptz` is is UNIX-style Epoch timestamp, that is an absolute point in time in UTC. It says nothing about the timezone because there are no time zones in that context.
The confusion arises because when you insert into a field you can insert "19:00 on the 29th March 2025 in London", but this is simply converted to UTC at insert time and the timezone is lost.
If anything, `timestamp` does represent a time in the "real world" and `timestamptz` represents a time in the universe.
If time zones are important to you, you need to record the time zone in a separate field. That is all. Where you store this depends on context, of course. You could store the user's current time zone in a profile and render all times in that time zone (psql and other clients do this automatically based on the OS time zone btw). Or you could store the time zone with each record if that makes sense, ie. you want to know what the wall clock time was for the user at the time the record was taken.
Only use timestamp (timestamp without time zone) if you really need local/plain datetime.
(Ideally, timestamptz would be called timestamp/instant, and timestamp would be called datetime.)
- Those with safety training for this class of foot-pointed firearms, who know how dumb shit looks like and that they should not do it;
- Those with officer training who are able to recognize the higher-level categories, and take principled approach to safety - e.g. recognizing that "date", "timestamp, "duration", "time of day", "time of week", etc. are different concepts and should not be mixed.