Many parts of the world move forwards and back a full _hour_ every year. We complain about it but it’s mostly ignored. A gradual drift of a couple of minutes is fine.
This gets very hard to reason about because Arizona doesn't _do_ daylight savings. Like, what happens when we request 24 hours of data from them, on the day daylight savings time flips over? Do we want midnight-to-midnight data, or do we actually want 24 hours of data, which might wind up timestamped differently since there's a duplicate 2AM one day and a missing 2AM another day.
A few years ago we had a contractor write some extremely messy code to handle cases like that, and soon it's going to be my job to try and refactor it into something readable.
I shudder to think how many bugs must show up for people who use right-to-left languages or use non-ascii charsets - especially before Unicode & emoji were popular. I’m ashamed to admit I don’t even know how to test if my software works properly in languages like Arabic.
Of course software made in the Bay Area assumes the whole planet uses pacific time. I’m sorry to throw shade, but that’s entirely in character for the area.
It's not just software. EVs struggle with cold and hot climates. They'll get better, of course. But with the engineers living in mostly temperate climates, conditions outside the development environment get less upfront attention.
Not in the Bay Area, but every other west-coast company I've been involved with is pretty militant about using UTC, typically due battle wounds from communication challenges or time-related bugs.
I would be very curious to hear which other major companies are deploying systems on local time in 2024.
UTC has leap seconds, unix timestamp doesn't. It's defined to have 86400 seconds per day, so there's no place to put a leap second. Instead, it either duplicates a timestamp or does "leap smearing" - slightly changing the duration of a second around where the leap second is.
Because DST might be a thing right now in that zone, but may no longer be next year, in which case the historical DST is needed to refer to times in the past.
Same for dates a bit further back when countries changed calendars and some days are missing.
So, we basically need a mapping of UTC -> local time as a function of time, and store this forever.
(For durations, we might need to have a mapping from TAI to UTC as a function of time, because a leap second messes with duration length. Smeared leap seconds are even worse in that regard)
You need to store it for certain applications, e.g. timetabling. if school starts at 9am, it starts at 9am local time, and if local time changes (DST, or more rarely a change in the time zone rules) then school start time runs with it. Or similarly, if a commuter train timetable has it stopping at a certain station at 7.05am, that’ll be local time in the local time zone.
So, to answer what I think you are saying - normally you divide the day into chunks (“periods”). At our children’s school, it is primary, there are only two notional periods a day (morning and afternoon), when our son goes to secondary school next year (which appears to use the same software) there will be several periods a day, one per a subject.
Anyway, in the system, a period is a class, not just in the academic sense, but also in the OO sense, and as such it has instances - “morning period” starts at 8.35 am local time any day the school is open. So that start time would be stored without a date, just a time plus time zone. But then, there is an instance of “morning period” every one of those days, which starts at a particular instant in time - today it starts at 2024-07-04T08:35+10:00. And yes, you could store that just in UTC, and convert to the school’s local time on display.
I suppose there are three main data types you really need: (1) date without time (2) local time in specific timezone (3) UTC instant
For (1), whether you need the timezone or not depends on the use case. For stuff like dates of births, you generally won’t know and don’t really need to know the timezone in which they were born. But, for other applications, it becomes important, since Wednesday afternoon in the Americas is Thursday morning in Oceania and eastern Asia, so whether it is Wednesday or Thursday depends on your timezone. People expect days to start and end at local midnight, not UTC midnight - which for me is 10 or 11 o’clock in the morning.
For hire dates, many jurisdictions have employment laws that have different rules depending on how long you’ve worked there. So you need to know how many days since hire to know what legal regulations apply to the employee. And obviously that is meant to be calculated in local time, if you do it in UTC or HQ timezone it could be a day out, which might cause legal issues. (e.g. in some jurisdictions it is easier to fire an employee in the first six months, you wait until the last day to terminate them, except because you got the day off by one, that was yesterday, now you have terminated them illegally)
Time zones are a curse on humanity.
The thing that's a curse on humanity is daylight saving time.
I have coworkers and friends in different timezones, and timezones do literally nothing but complicate coordination. Even if it's a small friction, they are 100% friction.
As far as I can tell, the main benefit of timezones is that it allows people within the same timezone to converse as if timezones don't exist. Which would also be the case if timezones didn't exist.
But there’s PT (pacific time), as well as PST and PDT (pacific standard and daylight savings time) if you need to be specific with whether or not daylight savings is happening.
EST, according to Google, is the east coast of America when daylight savings is not happening. For Sydney and Melbourne, you want AEST / AEDT / AET depending on if you want to specify Australian east coast standard, daylight savings or current time (which changes depending on the date).
It’s all hilariously exhausting to keep track of. Simple enough you think you can remember it, but complex enough you will miss your meeting even though you checked twice because the calendar was set to the wrong timezone and it didn’t matter until today. The relative time between Australia and California changes 4 times a year by 1 hour, depending on the local daylight savings time in both countries. I hate it.
Although AEST/AEDT are probably more common-in part to avoid confusion with American EST/EDT-people use EST/EDT for the Australian time zones too - random example: https://www.support.transport.qld.gov.au/qt/systemmaintenanc...
Some computer systems (probably designed by Americans) want timezones to have abbreviations but insist they can only have three letters, so in those systems the Australian timezones have three letters. I definitely remember seeing EST meaning UTC+10 on Unix systems before
edit: not trivializing missing the talk, i would be pissed.
So the timezone difference between US and Australia (or at least their DST using parts, since in both countries some states don’t observe DST) is 2 hours more in one half of the year than the other. And it changes four times. In January, Australia has DST and US doesn’t. Then in March US starts DST and moves one hour away. In April, Australia ends DST and moves another hour away (in the opposite direction). In October, Australia starts DST and moves an hour closer to US. In November, US ends DST and moves another hour closer to Australia.
Of course as soon as one of those assumptions is not 100% true, using UTC internally and local time for display is usually by far the better option (though there can still be difficulties there – time is never as easy as you'd think it should be).
The trouble comes when people use to working local only (timezone wise) slap together a PoC of something that might need timezone awareness, and don't fix the deficiency at any point as they progress through PoC->prototype->alpha->beta->v1. The longer you leave the change the harder it is to do, and it not being easy is why once something nominally reaches V1 (or often at first alpha release) such a fix is seldom ever made.
Timezone awareness has improved massively in recent years though. I think some cloud providers have accidentally helped there by defaulting to UTC (for instance all AzureSQL DBs default to UTC for everything, as do VMs and other things in Azure). Though here in the UK minor issues due to bad assumptions are still common when we transition back or forth between GMT and BST.
The way to avoid this pain is to just use UTC on day one, regardless of requirements. Hard and fast rule that everything needs to be UTC. Need to display it? Converting to local time is trivial.
Same thing for text. Use Unicode unless otherwise specified.
That can cause extra concerns for always-in-one-timezone systems where you might care where midnight falls (“did this event happen today or yesterday?” is a question that needs extra steps to answer, for instance). Nothing complicated, but extra work you might want to skip at the quick PoC stage.
But yes, beyond PoC work and all but the simplest other work, I'd agree with UTC all the way from the ground up.
Trying to make sense of them as purely time keeping artifacts is bound to have misunderstandings.
https://www.bbc.com/worklife/article/20170609-its-time-to-pu....
> The Spanish also go to sleep later than their European neighbours. According to Eurostat, Spaniards go to bed, on average, at midnight, compared to Germans at 10pm, the French at 10.30pm and Italians at 11pm.
We're the exception, not the rule.
Brazil, the largest country in South America, had DST until very recently (2019). A large chunk of South America observed DST at some point. Some stopped in the 90s, others stopped much more recently.
> Funny that you only consider North America as 'western'.
No, he's including Europe, which mostly observes a summer time offset.
Moving backwards means you (your code) experiences the same time twice.
So from my point of view, they were spot on
Are there timezone areas in the world which change their UTC differential fluidly throughout the year and by the yard to pin midday to the highest position of the sun?
I'll definitely say that that'd not only surprise me but blow my mind if true!
If there isn't ... Then what was your point?
there is always some drift between the suns position and midday
Your point is about as coherent as saying CO2 levels in the air aren't going higher, because the sea is blue