dealing with UTC dates and the future

Question

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?

Answer 1

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)

Answer 2

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.

Answer 3

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.