I just discovered, that storing dates in utc is not ideally correct if we are also dealing with dates in the future. It seems to be the case because, timezones seem to change more often than we think they do. Fortunately, we seem to have the IANA tzdb that seems to get updated periodically, but, confusingly, postgres seems to use a specific version of the db which it seems to have at build time..
So, my question is, if the timezones are changing, with daylight saving going on, with political, geographical adjustments happening, and our database is not with the latest of the tzdb, how would we be able to keep track of the accuracy of the dates in the system? Additionally, would libraries like date-fns-tz basically not be accurate to account for new timezone changes?
Ideally I would think a library would make a network call to a central server that would maintain the latest changes, but, it doesn't seem to be the case. How are the latest date/timezone changes usually dealt with?
The IANA time zone database collects the global knowledge about what time zone was in effect at what time in every part of the world. That information is naturally incomplete, specifically when it comes to the future. A (IANA) time zone is not an offset from UTC, but a rule that says when which offset from UTC is active. EST is not a time zone in that sense, it is an abbreviation for a certain UTC offset. If you live in New York, you will sometimes have EST, sometimes EDT, depending on the rules for the time zone America/New_York. Of course you should update the time zone database, but not because the timestamps change (they are immutable), but because the way that the timestamps are displayed in a certain time zone can change.
What is stored in the database is always an UTC timestamp, so the timestamp itself is immutable. What changes is the representation. So if you predict that the world will end next July 15 at noon Austrian time, and the Austrian government abolishes daylight savings time, your prediction will be an hour off (unless you expect the cataclysm to follow Austrian legislation). If you are worried about that, make your predictions in UTC or at least add the UTC offset to the timestamp.
If you store the timestamp with time zone in the database, and you query it today with timezone set to Europe/Vienna, you will get a certain result. If you update the time zone database, and the new legislation is reflected in the update, then the same query will return a different result tomorrow. However, it will still be the same timestamp, only the UTC offset in use will be different:
SELECT TIMESTAMP WITH TIME ZONE '2023-07-15 12:00:00+02'
= TIMESTAMP WITH TIME ZONE '2023-07-15 11:00:00+01';
?column?
══════════
t
(1 row)
To clarify #Laurenz's statement in the comments further with an example, lets take an extreme case of samoa , where they switched from GMT-11 timezone, to GMT+13 skipping an entire day.
While ignoring what a timezone actually is (different similar opinions in the comments), for the purpose of the calculations below, lets just consider it a value offset from the standard UTC. Also, do note, I use my own symbolic ways to calculate, but, it is very understandable, hopefully ;-)
so, samoa on Dec 29, 2011 skipped a day, how? Based on what I found, when the clock struck midnight they effectively skipped Friday. But, the unix timestamp
remains equivalent/unchanged:
GMT-11
(-)GMT+13
__________
= 24hrs
Let, WST=GMT-11
2011-12-29 T 24:00:00 - 11 (clock strikes midnight)
= 2011-12-30 T 00:00:00 - 11 (WST)
= 2011-12-30 T 11:00:00 (UTC)
now the switch occurs, WST=GMT+13
2011-12-31 T 00:00:00 + 13 (WST)
= 2011-12-31 T-13:00:00 (UTC)
= 2011-12-30 T 11:00:00 (UTC)
So, as far as I can see, storing future dates does not really affect the value of the date itself. But, what it does affect is the way the dates are displayed, e.g. if the timezone info was not updated, people would still see the day after the 29th at samoa as Friday, 30th. But, in that case, it would be Fri, 30th GMT-11, whereas if the information was updated, it would be Sat, 31, GMT+13. So, all is well.
more details in the comment section of #Laurenz's answer
Also, as #Adrian mentions above, softwares that deal with timezones, come packaged with a version of tzdb if they support the conversion at all. It seems to be the case in postgres as well though it seem you can configure it to use the system's version. For such cases, you gotta update the software or the system's db itself.
I understand that you want to store a future point in time, like "10:00am on July 5th 2078 in the time zone of Australia/Sydney", regardless of what offset that time zone has compared to UTC when you retrieve the point in time again. And when the time comes, the point in time might not even exist, because it is being skipped for the introduction of daylight saving time (or it might exist more than once).
Speaking XML Schema, the information you want to store consists of
a dateTime without timezoneOffset, in the given example 2078-07-05T10:00:00 (no trailing Z)
plus a time zone, given as a string from the IANA database, in the given example Australia/Sydney.
I don't know how this is best stored in a PostgreSQL database, whether as two separate strings, or in a special data type. The PostgreSQL documentation says:
All timezone-aware dates and times are stored internally in UTC. They are converted to local time in the zone specified by the TimeZone configuration parameter before being displayed to the client.
That sounds to me as if the UTC value was fixed, and the local time value in a given time zone might change if daylight saving time is introduced or abolished in that time zone. (Am I correct here?) You want it the other way round: The local time remains the same and the UTC value might change after DST introduction/abolition.
For example, assume that polling stations for the next general election open at 2025-09-21T08:00:00+02:00 in my time zone. But if my country abolishes DST before then, they will open instead on 2025-09-21T08:00:00+01:00 without an explicit rescheduling. In other words: The UTC time changes, but the local time does not.
Or consider a flight whose local departure time and time zone are stored, which has a duration of 10 hours and arrives in another time zone. Its local arrival time then changes when the offset of the departure time zone changes, for example, because daylight saving time is introduced or abolished in that country on day X, but the offset of the arrival time zone does not change. An app that computes the local arrival time must then show a changed arrival time when it is executed on day X or later, although the stored data (the local departure time, departure time zone, arrival time zone and flight duration) have not changed. The required change can happen automatically if the app uses a library that is based on the IANA time zone database and receives an upgrade that includes the DST introduction/abolition before day X arrives.
For an example of such a library, see https://day.js.org/docs/en/timezone/parsing-in-zone.
I'm developing a client and server application and I'm using luxon and postgres.
Considering the current time i'm writing this post (2022-11-15T22:55:27.374-03):
On local server/db, the dates are saved on UTC-3, like the above (2022-11-15T22:55:27.374-03);
On hosted server/db, the dates are saved on UTC-0 (2022-11-16T01:55:27+00);
My problem is that i need to query data in between begin and end of day, and while on local it works, on server it doesn't because day starts at 03:00 and finishes on next day 02:59.
I tried hardcoding conditional offset based on env but i dont believe that this should be the best solution...
Is there a proper way to handle this timezone difference?
You need to define what beginning and end of day mean. There are a total of 48 hours in which any timezone can be in a particular date. You'll have to decide if your queries will be "today" in the server's timezone or in the local client's timezone. If your queries are the server's "today". You can parse any request as plain timestamps and let the receiver decide what bounds to use. The flip side is to request the actual date string you want e.g. 2020-01-01 and let the server calculate the bounds in its timezone.
We have two different cassandra cluster on two different timezones.
Cluster1: 2.1.8 version, with IST TZ
Cluster2: 2.1.9 version, with UTC TZ
On cluster1 for a select query with timestamp column, i need not mention the tz[+0530] value , whereas on the other cluster I must and should provide the TZ value in select query to fetch the row. Is it to do with cassandra version?
I use cqlsh to do the query part. I tried cqlshrc file option, which only changes the format of output.
cluster1:
select * from test.check where row_timestamp = '1970-01-01 00:00:00';
cluster2:
select * from test.check where row_timestamp = '1970-01-01 00:00:00+0000';
IF no TZ is mentioned, i get "0" rows.
I dont want to give TZ in cluster2, please adivce how to do that.
It is a bit strange, I must admit, but there might been some changes in time zone manipulation between 2.1.8 and 2.1.9. This is from the changelog:
(cqlsh) Fix timestamps before 1970 on Windows, always use UTC for
timestamp display (CASSANDRA-10000)
On the other hand, the documentation is quite clear on this issue:
If no time zone is specified, the time zone of the Cassandra
coordinator node handing the write request is used. For accuracy,
DataStax recommends specifying the time zone rather than relying on
the time zone configured on the Cassandra nodes.
So, my sincere recommendation is to specify the time zone, and specify the same, presumably GMT (or UTC time). Save yourself the headache. Mind, GMT is not exactly equal to UTC, there is a slight difference in meaning. That way, you should ignore the time zone settings on the clusters. The time stamp is ultimately stored as a number of milliseconds (from certain point). The time zone information is purely a "rendering" thing. The number of milliseconds passed is the same in, for example 2015/03/05 14:00:00+0100 and 2015/03/05 16:00:00+0300.
If you are specifying nothing, and getting 0 results, while you do get results when you use +0000, then make sure that the data you are expecting originally is written with the expected time zone. Maybe there actually is not any data in the span because of that, or the coordinating node time stamp is different.
I need to run a Cassandra instance on Windows... Don't ask why...
Anyway the issue is that I have time stamp columns that show datetime in PST time zone but I would like to see GMT time zone. My machine runs with BST time zone(British Summer Time).
Is there a way for me to change the default time zone to GMT?
Timestamp values are stored independently from the timezone they have been converted from. Any representation of a TZ will be done by the cqlsh which is depending on Python for the conversion from the TZ agnostic timestamp value to the cqlsh output. Python in turn will use the OS default TZ. In Linux, you can change the TZ by setting the following environment variable in the same shell used to start cqlsh: export TZ='GMT'. I suppose this should work for Windows as well using something like set TZ='GMT'.
Update 11/Feb/2016: described behaviour will not work anymore for 2.1+. See this answer for details. (The linked answer explains this has been fixed.)
i want to convert UTC date time to local date time by myself and do not want to use .net TimeZoneInfo or other classs about this.
i know Tehran is a GMT offset of +03:30 i use code below to convert UTC Date time to tehran (my local computer is in this location):
DateTime dt = DateTime.UtcNow.AddHours(3.30);
it shows time like 5/2/2014 8:32:05 PM but Tehran time is 5/2/2014 9:32:05 PM it has one Hour deference.
How can i fixed it?
i know Tehran is a GMT offset of +03:30
Well, that's its offset from UTC in standard time, but it's currently observing daylight saving time (details). So the current UTC offset is actually +04:30, hence the difference of an hour.
I suspect you're really off by more than an hour though, are you're adding an offset of 3.3 hours, which is 3 hours and 18 minutes. The literal 3.30 doesn't mean "3 hours and 30 minutes", it means 3.30 as a double literal. If you want 3 hours and 30 minutes, that's 3 and a half hours, so you'd need to use 3.5 instead. The time in Tehran when you posted was 9:46 PM... so I suspect you actually ran the code at 9:44 PM.
This sort of thing is why you should really, really, really use a proper time-zone-aware system rather than trying to code it yourself. Personally I wouldn't use TimeZoneInfo - I'd use my Noda Time library which allows you to either use the Windows time zones via TimeZoneInfo, or the IANA time zone database. The latter - also known as Olsen, or TZDB, or zoneinfo, is the most commonly-used time zone database on non-Windows platforms.