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Comment by benreesman | Hacker News Reader

Tangential, but man time zones are just really hard. Some theoretically “simple” rules seem to routinely violate human intuition, and the almanac actually changes based on laws and stuff.

Keeping everything in Zulu ms or ns and rendering into timezones as late as possible is a useful guideline as far as it carries one, but sometimes it’s not enough, this is one of a zillion reasons why e.g. calendaring is just really hard.

I’ve long since mentally tagged “timezone handling”, in any language, as a slow zone: let’s stop and really think this through.

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It's even harder. Is the time the user entered actually an always-local time?

As in, if I set my alarm for 08:00, I'd expect it to wake me up 08:00 local regardless if I'm in Oslo or I've traveled to say New York.

However if I make a calendar event things are less clear cut. For meetings and such it makes sense that the time is global time (hence stored in UTC), but for tasks it can be be either or, depending.

I really like what google calendar does, which is to allow you to display both.

I was baffled when I first entered my schedule for a conference into Google Maps, only to find when I flew across the country that all the times were wrong. I understand that it's a reasonable choice to use the local time zone at event creation, but it sure wasn't what I expected (I vaguely thought it would be like your alarm clock example, where it would move with the local time).

I once made a flight by the slimmest of margins possible (the gate crew stopped closing the check in because they saw me sprinting through the terminal hall), because of this and only one hour difference between where I started from and where I went.

I mean obviously wherever a date time entry can be found, there should be a TZ drop-down and a DST checkbox.

lusers who don’t know what those things are will learn soon enough.

I really mean it. The ambiguity is a problem.

I've done the same.

Google did 100% the correct thing, but it goes to show that this is just an inherently tricky thing not limited to whether you are writing code.

Thank you for the examples - now I know to not try to make and publish an alarm clock or calendar app/site/feature...

What happens exactly if you set your alarm for 1:30AM on DST changes. Does it go off twice in Autumn and skip altogether in Spring?

The whole DST time change is predicated on the fact that almost nobody sets alarms (or does anything time-sensitive) at 130AM local time. So yeah, if you do that, that’ll happen.

Is that actually true though? Does, say, an iPhone actually fire the alarm both times the clock flips to 1:30am? I tried Googling it but I could only find info about missing alarms entirely when springing forward. I also found some info about how cron works but even then the answer was “it depends which version of cron you’re using.”

You don't even need timezones or DST to run into issues along these lines.

Let's say you have have a repeating 7am alarm on your phone. What should happen if you want to set it to 8am for one day? I mean, without walking you through a dialog that looks like: do you mean today 8, or tomorrow 8?

This is the part that isn’t even technically hard per se. It’s just hard because there’s literally no way to know what the computer should do because there’s no way to know what a human would even expect.

In many cases reasonable assumptions can be made, e.g. a calendar event for two people in different time zones to meet on a video call almost certainly needs to be scheduled for a proper time (or likely UTC, which is close enough since if you can ignore leap seconds). Although even then I wonder what the two attendees would expect if, between the scheduling of the event and the event itself, both of their local time zones changed their DST rules such that their UTC offset changed by +1 hours. If only one time zone changed its UTC offset we might assume that the previously agreed upon UTC time would hold, but it’s hard to say what the humans would expect if both of their time zones shifted the same way.

I've been in this situation, and what people expect are that the "main office" time holds, even for remote teams that should know better.

Then again, you don't want all your meetings changing by an hour half the year to avoid a one-week discrepancy.

If there’s a clear “main office” that’s probably a reasonable assumption, and it’s probably a reasonable feature for calendars to explicitly allow scheduling an event for a specific time zone (instead of converting every datetime to UTC or similar before storing it, which is the usual recommendation among programmers).

I'm pretty sure that both MS and Google default to the time of whoever set up the meeting.

I think it's fixed now, but DB (german transit company) used to always use your local time in its app, making it incredibly confusing to book tickets or even check on tickets you already have. I planned a trip from Japan and the train times showing up were something like 2:30 in the morning.

Another bad example is using Windows bootcamp on a mac. Windows writes local time to the RTC and MacOS writes universal time, so when switching between MacOS and Windows, it's always wrong at first.

> Windows writes local time

I seem to recall there's a registry key to control that and have it store UTC to the RTC

edit: found that one https://superuser.com/questions/975717/does-windows-10-suppo...

I do this, If you have a unix install on the same host, It really helps keep a sane time config. there are ways to do it the other way around. sysctl kern.utc_offset on openbsd. But the unix way feels better to me, so I change windows.

> But the unix way feels better to me, so I change windows.

I'm with you as well. Sometimes you don't have a choice when dual-booting, MS-DOS only wants to work in local time while on the opposite end of the spectrum MacOS only wants to work in UTC, but if I have the choice I'm going to change Windows to use UTC.

Good to know, thanks!

Even worse with transit in Vienna/Austria where if you buy tickets in advance for a specific day or date range they seem to take your local time into account if those tickets are bought while in another timezone. That's for tickets where time shouldn't even matter because they are for full days (from midnight to midnight).

If you're in Pacific time and you buy the ticket for a date in the future (let's say June 1st) before 3pm your local time everything works fine. But if you buy the same ticket for June 1st after 3pm Pacific local time (which is already the next day in Vienna) the ticket that you're buying isn't for June 1st but actually for June 2nd (only visible once you receive the confirmation).

So apparently they're somehow translating between timezones for the displayed dates, although that doesn't make sense: If I buy a ticket for June 1st on Vienna public transport I don't care what my local time is. Even if I'm in California while buying that ticket I still want it to be valid on June 1st instead of June 2nd.

> For meetings and such it makes sense that the time is global time (hence stored in UTC)

I think this is tricky too - if you have a 9AM meeting, if your time zone switches, should the time of your meeting change too? Most of the time, no, but if you have a meeting with other people in far away time zones, maybe you don't want it to change.

Doesn’t “meeting” imply multiple people, and the only time you’d need to worry about time zone issues is when those multiple people are not all in the same time zone?

Some people using "meeting" to block off time for the sole purpose of being unavailable for someone else to schedule you in a meeting.

That's hard enough for a calendar program to figure out, but also it can also be affected by whether other meetings involved multiple time zones, especially when something is recurring.

> the almanac actually changes based on laws and stuff.

some of my perennial favorites:

- in samoa, there was never a december 30th 2011.

- in indiana, time zone observance is determined per county

- in arizona, the reservation land observes dst, and the state doesn’t, and reservation lands can enclose non-reservation lands.

- lebanon recently had a legislative dispute about dst such that some entities in the nation did or did not observe dst for about a week.

> - in samoa, there was never a december 30th 2011.

That was interesting, so I had to read about it. Apparently they skipped a day to be closer to Asian time zones. They did the same thing in the opposite direction in 1892 by having two 4th of Julys to be closer to American timezones. Now they kinda just walked back on that.

Interestingly I think the Python datetime approach can handle both of these with no problem. The rest of your list sounds... a lot tougher.

If you're storing historical dates, September 1752 only had 19 days.

oh, historical time is an absolute mess. i have yet to find a library that handles BC dates except maybe astropy — and they have different priorities!

The transition from the Julian to Gregorian calendar also happened on vastly different dates in different countries. Most of Western Europe changed in 1582, the British Empire (and its American colonies) in 1752 as you said, and other countries switched even later like Russia in 1918.

> Zulu ms or ns

Is that timestamps as in unix timestamps? That won't work with birthdates for people more than 53 years old for example; a colleague mentioned a weird issue for someone born in 1938, but that was because the timezone in that location at that time was something like GMT+1:23, then the Germans came around and standardized it to +1:20, then later it was normalized to +1:00.

tl;dr, time and date can only be accurate if time, date and location are known. Actually, some places have moved to different timezones over time as well, so you can't just say "this time has a timezone of GMT", you have to ask "what is the timezone of this location at this time/date"

To get to/from that local time representation, you can still store it as a timestamp (to know when it actually happened) and use a timezone database that tells you what the people living there in that moment labeled that time as, right?

You still have the fundamental issue of storing it correctly in the first place. Do most languages time libraries account for localized temporal shifts of this nature by default? It does seem true that you could query an authoritative source, but on entry:

If I say the event happened 60 years ago exactly, and it’s 1pm, what do I mean by “exactly?” Do I mean the day 60 years ago, at 1pm? Or 60 years ago, taking into account timezone shifts? Do I, the person entering this information, even know or care about this distinction? If you enter an event as 60 years ago, would your average joe expect that the time would shift by 23 minutes, or would they think there’s something wrong with the software?

What about timezone differences that vary depending not only on geography, but also on the ethnicity of the people associated with the event?

You mean ethnicities that are still deprived of nationhood, so the empire they are subjects of assigns different timezones to them compared to other ethnicities they rule ?? (Any examples ??)

The classic example (and it may in fact be the only extant one) is the occupied West Bank, where Israelis keep a different time than Palestinians (sometimes): https://www.972mag.com/the-worlds-only-ethnic-time-zone/

Yeah, ISTR that there are (or have recently been) some at least theoretical (that is, where there were political claims with different timezones, but not necessarily populations in the affected area using both) ones besides that, but that may well be the only current one.

There's some informal use of a Xinjiang timezone among Uyghurs. China as a whole only has one official timezone.

[1] https://www.timeanddate.com/worldclock/china/urumqi

[more background here] https://www.theatlantic.com/china/archive/2013/11/china-only...

> What about timezone differences that vary depending not only on geography, but also on the ethnicity of the people associated with the event?

Typically, recently, this problem would occur in disputed territories where the “legal time” at a certain point differs because it is claimed by different polities with different timezone rules, so its more of political allegiance than ethnicity (though those correlate).

You are correct but also don't have to look that far: in Indiana USA, different counties have different daylight-saving rules, so "August 13 1pm" is a different time depending in which county you are (or live, or work).

> Do I, the person entering this information, even know or care about [the time of the event I'm referring to]?

I'm sorry but if you don't even know yourself what you mean, the computer cannot know it either. The topic here is how to store a datetime, not how to guess meanings.

> What about timezone differences that vary depending not only on geography, but also on the ethnicity of the people associated with the event?

Again, display conversion. Have these natives put their date system into the timezone database and make the UI have a selector for this timezone. Under the hood it still happened at a certain actual moment in time which is what should be stored in the database.

No significant difference from any other timezone that you have to apply to a unix timestamp to get a display time, or conversely, apply to input data to get the timestamp to store.

I'd have yet another interpretation: the exact time when Earth needed precisely 60 revolutions around the sun to reach its current location.

For things in the past, Zulu is fine. But you won't know Zulu for a date in the future, because Timezone changes may happen.

So Zulu for the past, datetime+location for the future.

Time zones, character encoding, localization. I admire people who dig in and really understand them deeply, but as far as I’m concerned they are all exposed high voltage wires that I avoid going near.

> I’ve long since mentally tagged “timezone handling”, in any language, as a slow zone: let’s stop and really think this through.

I will second you ok that: date & Time handling across timezones was one of the 2 hardest things I've ever worked on that are deceptively simple on the surface. The other was adding localization/making an app translatable to any language after shipping the first version with hard-coded text.

Same here. Worrying about current timezones is a hard enough problem. Getting accurate historical timezones handling working is an even harder problem. Getting consistent results across Windows, Mac and Linux is an even harder problem. Needed it for proper handling of historical time series for financial data.

Non-programmers don't care about this nearly as much as we do. The prevailing attitude is "Ok sure there are edge cases, but so what? My use case is simple".

And after thinking about it a lot more, I think this attitude is actually more sensible than mine used to be. Why do we as programmers keep trying to fit a square peg into a round hole? Timezones are a political construct, not a mathematical one.

We should address edge cases by deferring to people as much as possible. Oh, you find it annoying to enter an offset? Don't worry about it, it's optional! Your dentist will expect you for your cleaning at 8:00am +/- 5 hours.

If we keep smoothing over the rough edges for our users, the problem will only get worse. Better to make timezones as painful as possible for as many users as possible. They'll eventually vote for something less obnoxious.

As much as I would love this too from a programmer's perspective, reading [1] convinced me that time zones are a necessary evil we just kinda have to deal with. They could be improved by a lot, though. First thing I'd do is get rid of DST everywhere.

[1]: https://qntm.org/abolish

Thanks for the reference. I have a number of issues with that essay, but fundamentally, once we have people on Mars we'll have exactly this issue. There's no easy way around it and we'll always have to negotiate hours that work for both parties (which is what I basically have to do with my family who are about 90º phase shifted from me).

> We should address edge cases by deferring to people as much as possible. Oh, you find it annoying to enter an offset? Don't worry about it, it's optional! Your dentist will expect you for your cleaning at 8:00am +/- 5 hours.

> If we keep smoothing over the rough edges for our users, the problem will only get worse. Better to make timezones as painful as possible for as many users as possible. They'll eventually vote for something less obnoxious.

This is gonna go over great with the people making purchasing decisions.

Implement everything in Zulu, then watch as the frontend ticket for proper rendering stays on the bottom of the backlog, because you always have more concerning problems than users having to do mental math on times.

Are you giving this as genuine advice, or are you suggesting implementing time zones early instead as a form of penance?

I agree. Local time with timezone is really a presentation-layer concern.

For server-side and storage layer, it's saner for everyone to simply store in UTC or epoch seconds/nanoseconds.

Time zones are themselves a footgun. There is no actual reason for morning to start at approximately the same time everywhere. 9am can be 9am everywhere. In some places 9am can be sunrise, in some it can be sunset, and in some it can be the middle of the night. No conversions, no confusion, no resetting of watches...

We don't change the months in the southern hemisphere. Australia is cold in July, Europe is cold in January, this doesn't seem to bother us.

It's a lot easier to remember "north and south are opposite" than it is to remember, "Oh right, 9 am is morning in England, early morning in New York, and the middle of the night in Los Angeles." And so on for every possible time of day.

Whereas it's (relatively) easy to guess, "Los Angeles is in California, that's three hours behind me in New York, so it would be 6 am there at 9 am here, so it's too early to call them yet." Not perfect, but not too bad for simple cases, and easy to do in your head most of the time.

Timezones are about standardizing _human_ time, not actual time.

> There is no actual reason

There may be no technical reason for it, but having a common reference point for times of the day in terms of human experience and behavior is an actual reason. Saying you were walking down the street at 2am or 3am is immediately distinct from saying you were walking down the street at 10pm or 11pm in ways many people understand. Having to qualify it with the date and geographic location and/or sunrise or sunset patterns would be a much bigger PITA than your average person will ever experience because of time zone inconsistencies. It's just not that important for most people to be able to pinpoint a timestamp to one exact time throughout the world.

We've got GMT. Nothing is stopping anybody from using that to schedule their lives. There's a reason people don't.

> Tangential, but man time zones are just really hard

That can be expanded. Time and dates is just fundamentally hard.

I think keeping time in UTC as primary data for as long as possible is a really bad idea, as explained by Jon Skeet quite a while ago: https://codeblog.jonskeet.uk/2019/03/27/storing-utc-is-not-a...

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