8 ms·

I think this might be one of those “difficulty curve” memes. On the left, the Padawan: “just use UTC timestamps”. In the center, the journeyman: “no!!! we have

• echelon 2y ago
  This is the language of your engineering team, but not the language of your customer.
• gvkhna 2y ago
  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.
  • closewith 2y ago
    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.
  • nafey 2y ago
    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.
    • tisdadd 2y ago
      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.
    • moqmar 2y ago
      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).
    • Denvercoder9 2y ago
      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.
      • nafey 2y ago
        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.
  • netsharc 2y ago
    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...
    • mminer237 2y ago
      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.
      • netsharc 2y ago
        Yeah, these are workarounds we have to use because many pieces of software weren't implemented fully...
      • lxgr 2y ago
        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.
• Etheryte 2y ago
  While UTC timestamps look like they solve all of the problems in one passing, they have plenty of edge cases that can blow up spectacularly. A good example of this is daylight saving time and countries changing their rules around it. You would think this is a rare occurrence, but it actually happens pretty often [0]! In the last ten years, fourteen countries have abolished daylight saving time. [0] https://en.wikipedia.org/wiki/Daylight_saving_time_by_country#Past_observance https://en.wikipedia.org/wiki/Daylight_saving_time_by_countr...
  • RandomThoughts3 2y ago
    Technically you could use TAI for most things internally and use an offset when you display. It works perfectly for records. The only issue is with scheduled things when your users probably want to point at something in their timezone.
    • labster 2y ago
      The main problem is that future values of TAI converted into most time zones is nondeterministic. On the other hand, at least you can guarantee all TAI seconds will actually occur.
    • ikekkdcjkfke 2y ago
      Or toLocalDisplayString(TAI timestamp). It seems like timezones and their rules change over time and geographic location, using todays rules to show dates back in time with previous rules would be broken right?
      • jitl 2y ago
        Well, any correct time handling library will use something like the IANA timezone database (aka tz/tzdb/zoneinfo) which stores all the changes in timezone rules for localities over the years - a library that only knows "today's rules" would be fundamentally broken as you say. So the internals of toLocalDisplayString(timestamp, timezone, calendar, locale) should look up the appropriate rules in effect at the timestamp and use those to decide the local time to display.
• happytoexplain 2y ago
  No - the master's answer is that you use one or the other depending on whether you are representing absolute time or clock/calendar time. The expertise lies in understanding the difference between those two things, which one your use case falls into, and how to ensure other developers and the very APIs provided by your platform do not make silent assumptions about that decision. Often it feels like you can "just use UTC" because it's hard to imagine the fail states, and APIs often make tz assumptions for you so you may never notice those fail states (occasionally your software will just be buggy and annoy a user, decreasing the feeling of quality and trust - it may never result in a disaster).
  • naasking 2y ago
    There is no such thing as absolute time so the distinction you're trying to make is not clear.
    • langcss 2y ago
      Most use cases don't care about relativity
  • SoftTalker 2y ago
    And even then it's difficult. Time zone definitions change. Summer time does not start and end on the same date everywhere. So you also might need to know the UTC offset corresponding to the time zone at that time not just the UTC offset of that time zone today (assuming it still exists) Edit to add: still, this sounds like a great improvement. A common mistake that naive programmers make is to believe that they can hand-roll their own date/time functions. And JavaScript sort of forces them to do that. See also Falsehoods programmers believe about time at https://gist.github.com/timvisee/fcda9bbdff88d45cc9061606b4b923ca https://gist.github.com/timvisee/fcda9bbdff88d45cc9061606b4b...
    • treflop 2y ago
      It’s not difficult if you understand what you are trying to do conceptually. For example, if you are trying to say “run this task at this specific time in Los Angeles time” and you put a specific UTC offset, it just doesn’t make any sense. Los Angeles time consists of two offsets, and yet here you openly put down something wrong. You need to put into your system literally Los Angeles time (like America/Los_Angeles) because that’s what you want. You didn’t want an offset. Dear god why are you putting an offset then? Or you want to define a specific date but then you put into your system a date with a time like 12/12/2024 00:00. Like you started off with “I want to define a specific date” and here you openly just put a time. You had to make up 00:00. Why are you just doing random things without thinking?
• stefan_ 2y ago
  Sounds like you are in the middle. Because the Jedi master knows you need both.
• lexicality 2y ago
  Dates of past events are always UTC, dates of future physical events must always be a timestamp and a location, dates of future virtual events are either UTC or location based depending on how virtual they actually are. I can tell you from bitter experience that if you have an event where a person has to arrive at a place at a time and you store that as UTC instead of wall clock, you will have a problem at some point
  • mirekrusin 2y ago
    Past/future doesn't matter, what matters is context. You don't need to use timezones for ie. setTimeout even though it refers to the future and you should have timezone available for past events ie. atm withdrawal.
    • marshmellman 2y ago
      setTimeout takes a duration in ms, not a timestamp. If it did take a timestamp, I think you would need to pass it a timezone to disambiguate. Maybe a better argument is, “when you’re setting a phone alarm, you don’t tell it a timezone.” Maybe the distinction is whether the timezone is established at write time or read time.
      • jitl 2y ago
        Scheduling on a calendar (occur on date X, time Y, in zone Z) and scheduling based on timedelta from instant time (occur exactly X milliseconds before or after instant Y) are both valid, and you can do timedelta scheduling in UTC without needing timezones. The issues come from conflating the two; using timedelta reasoning for calendar scheduling, or using calendar scheduling when you want an exact timedelta.
      • lxgr 2y ago
        > when you’re setting a phone alarm, you don’t tell it a timezone. Which coincidentally and ironically makes phone alarms surprisingly difficult to implement in a way that does not break in the face of DST shifts, timezone changes etc., as Apple has learned the hard way a couple of times.
• lxgr 2y ago
  > the Jedi master: “just use UTC timestamps”. That poor Jedi master is going to miss the recurring workgroup meeting at least twice per year though, after either DST switch :)
• crabmusket 2y ago
  In my experience the Padawan's answer is actually "just use my personal local timezone and don't think about the needs of users elsewhere". Maybe I'm thinking of a group with less experience than you had in mind, but novices do not even know about UTC.
• taspeotis 2y ago
  Sadly UTC doesn’t work with future timestamps because we don’t know if timezones will be adjusted.