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Comment by gvkhna | Hacker News Reader

Yes that works for most use cases but there are use cases where you may need to store or shuttle the time zone. For instance you want to know this UTC timestamp was originally created in PDT. You would have to store two variables. Most other languages have this functionality it can be useful and is good to have, probably only needed by Jedi’s too.

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The article gives an example, of you buying a coffee with your credit card while travelling to Sydney, and returning to Madrid, and a few months later seeing a charge for 3:50 AM on that date...

Google Photos also get confused with "When was this picture taken?", my older model camera just stores the EXIF date in "local time" and I have to remember to change its timezone when travelling, and if GPhotos can't figure it out, it might show pictures out of the airplane window, and then the next series of pictures are from the departure airport because they're from a "later" hour (since it's missing the timezone info).

I suppose I could keep it at my home time or UTC...

That is how I would expect a bank statement to read though. I would find it infinitely more confusing if I bought something online in my bank showed the time of wherever the seller was located.

The photos problem is harder, but the app needs to just convert it from local time to UTC when you import it. There's not much that can be done if you take photos on a camera with a different time zone than you're in without more metadata.

Yeah, these are workarounds we have to use because many pieces of software weren't implemented fully...

You'll find that most bank systems avoid any notion of time precision higher than calendar days for a variety of reasons :) As a side effect, this practice conveniently avoids that problem entirely.

> That is how I would expect a bank statement to read though. I would find it infinitely more confusing if I bought something online in my bank showed the time of wherever the seller was located.

When using my banking app abroad (one that does show timestamps), I'm usually much more confused by their presence than by their absence.

> The photos problem is harder, but the app needs to just convert it from local time to UTC when you import it.

But I usually want to see the time in local hours and minutes for photos! Sunsets, new year's fireworks etc. happen according to local time, not UTC or my current timezone's offset.

Yeah, storage needs to be implemented in UTC and display needs to be in local time.

But which local time?

Sometimes, the local time at the place the photo was taken can make more sense, but it's not a general rule.

Exif (fun exercise: find out who specifies it and when it was last updated!) actually didn't even have a way of storing timestamps in UTC or with zone information until fairly recently: It was all local times without zone information.

I've seen some software work around that by combining the local date/time with an embedded GPS tag, if present, which does contain the time in UTC.

Ironically, time zones are a hack with bad precision about the position of the sun in the sky. The GPS coordinate side steps the nonsense (can be converted to the time zone as defined in that place at that moment).

And if your spouse is checking the charges from Madrid, they probably want to see it in Madrid time. There is no single correct answer.

I have always wondered why breaking the timestamp (in UTC) and the timezone into two separate data points and storing both is not the accepted solution. It appears like the cleanest solution to me.

From my experience, it certainly is. Easy to tell time in sequence as well as local time then. When daylight savings hits you can still calculate well, and can quickly get real time for hours worked out drive time for freight across time zones to follow the hours of service rules.

Two different implementations might make two different local times out of that, e.g. due to not being aware of changing DST/timezone policies. Hence the recommendation of separating between user/clock/calendar time (which must be exact to a human) and absolute/relative timestamps (which must be exact to a computer).

You can't accurately convert future local times to UTC timestamps yet, as that conversion changes when timezones change.

Let's say we schedule a meeting for next year at 15:00 local time in SF. You store that as an UTC timestamp of 2025-08-24 22:00 and America/Los_Angeles timezone. Now, imagine California decides to abolish daylight savings time and stay on UTC-8 next year. Our meeting time, agreed upon in local time, is now actually for 23:00 UTC.

Wow thanks for sharing this, this certainly is a use case not covered by the system I proposed. I imagine this will require versioning of the timezone so we can translate our America/Los_Angeles v1.0 timestamp to America/Los_Angeles v2.0.

You can make the same meme about this use case, too. One you get to the right, you realise you want two variables for this.

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