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Palestine switched off Daylight Saving Time this year with only 4 days notice. Anyone involved with bookkeeping and synchronizing should already know to never

Palestine switched off Daylight Saving Time this year with only 4 days notice. Anyone involved with bookkeeping and synchronizing should already know to never attempt to handle time conversions using your own program logic updated by hand, because the definition of time is constantly changing and there are already projects like tzdata dedicated to centralizing the handling of it. There have already been 5 time zone updates this year, 4 changes to the date that Palestine switches daylight savings (one of them with only four days notice!) and Fiji decided to quit using daylight saving time, plus the Leap Second announced on July 9.

yyyk 4y ago
Reminds me of this story: https://darwinawards.com/darwin/darwin1999-38.html https://darwinawards.com/darwin/darwin1999-38.html
alerighi 4y ago
That is a different problem. You usually work with timestamps and then convert the time to local time only when displaying it or doing calculations that depend on the particular timezone. So it's less of a problem, since at lower levels you don't have the concept of timezone (you may never have it, for example on embedded devices). But if UTC shifts, it's another story. You assume the UTC (or UNIX) time to be a monotonic counter, and usually it's used this way, for example you compare two timestamps to know what definitively happened before another thing. Surely you don't take into account leap seconds... this is the problem. Leap seconds wouldn't be that much of an issue if the time is only increased, you have one more second, it's the higher level implementation that has the concept of time to (maybe) deal with it. But they become an issue if they can take behind the UTC clock since it can generate all sort of strange bugs. It's even a worse solution to make the duration of a second in a day longer or shorted, because you preserve the counter value, but you no longer can rely on the fact of 1 seconds being 1000ms long! By the way the concept of leap second is nonsense to me. What is the purpose? If the purpose is to keep in sync the time with the rotation of the earth, nobody will ever notice an error of a couple of seconds, or even minutes. It makes more sense to take the current definition of a second, and then wait till the error is noticeable to an human being (let's say an error of 15 minutes, that would take centuries and we will probably far gone from the planet) and adjust it by shifting all the timezone for that amount of time.
- elfchief 4y ago
  Actually, UTC -is- continuous. Even with leap seconds. At no point does UTC go backwards, at no point does a second happen twice, and at no point does a second change in length -- your 'extra' second happens as 23:59:60; when there's a negative leap second, the last second of the day is 23:59:58. UTC is not expressed or defined as an offset from some time in the past. The real problem for computing is when UTC is converted to unix epoch time, which is defined as an offset from the past, and by definition has exactly 86400 seconds in a day, every day, so some provision has to be made for those extra (or missing) seconds. And -that- is where the problem happens. But it's not UTC that's mucking around with the definition of time, it's the standard representation of time that's used in modern computing that causes the problems. That being said, UTC is still probably to blame for most of the problems, because it effectively requires knowledge of more than just a timestamp to understand when something actually happened. And that extra knowledge (the map of when leap seconds have happened before) changes often and irregularly. Epoch time could totally be redefined to include leap seconds, and that would solve lots of problems, but there's no practical way to distribute that updated leap seconds table to every system that would possibly need it...
- a9h74j 4y ago
  > and adjust it by shifting all the timezone for that amount of time Does this follow as a question?: Shift the timezones geographically, or in terms of how they all define [true monotonic] to local-time translation?
deleted 4y ago
[deleted]
akira2501 4y ago
That would most be a local problem and would not continue to exist for a long period of time. tzdata is complex, but having waded through it a few times, it seems needlessly so for the desired use cases. There is more editorial comments than data in the files, and they spend a lot of time and effort creating entries for historical time zones so that you can convert times in the year 1942 into territorial time zones that don't even exist any more with perfect accuracy. Except, even then, it seems like it's not. If you dig into the time routines that use tzdata, you see they must use an unstable conversion process that _may_ converge, but has a simple counter protecting it in the case that it _does not_ converge and in that case it just gives you a best guess time that may still be wrong anyways. Really.. the problem I have and I suspect is the majority of cases, I have a time in Some/Zone and I want to tell someone in Another/Zone what the time would be on their clock, today, right now. The time our grandfathers would have used during the War in summer is never going to be material to me in this instance or for the majority of use cases. This isn't meant to be overly critical of tzdata just a recent annoyance I experienced when trying to find a minimal code and cross language solution to the above problem.
- nightpool 4y ago
  > If you dig into the time routines that use tzdata, you see they must use an unstable conversion process that _may_ converge, but has a simple counter protecting it in the case that it _does not_ converge and in that case it just gives you a best guess time that may still be wrong anyways. What does this mean? Can you explain in a little more detail, or link to somewhere I might read about this? I haven't used tzdata myself, but I understand it in broad strokes as a list of time zones, and a list of which offsets apply to which timezones at which given dates. What am I missing?
  - akira2501 4y ago
    See the code for __mktime_internal in glibc, specifically the case for which it returns EOVERFLOW; but in general, the whole file has many comments worth reading. https://code.woboq.org/userspace/glibc/time/mktime.c.html https://code.woboq.org/userspace/glibc/time/mktime.c.html

yyyk 4y ago

Reminds me of this story: https://darwinawards.com/darwin/darwin1999-38.html https://darwinawards.com/darwin/darwin1999-38.html

alerighi 4y ago

That is a different problem. You usually work with timestamps and then convert the time to local time only when displaying it or doing calculations that depend on the particular timezone. So it's less of a problem, since at lower levels you don't have the concept of timezone (you may never have it, for example on embedded devices). But if UTC shifts, it's another story. You assume the UTC (or UNIX) time to be a monotonic counter, and usually it's used this way, for example you compare two timestamps to know what definitively happened before another thing. Surely you don't take into account leap seconds... this is the problem. Leap seconds wouldn't be that much of an issue if the time is only increased, you have one more second, it's the higher level implementation that has the concept of time to (maybe) deal with it. But they become an issue if they can take behind the UTC clock since it can generate all sort of strange bugs. It's even a worse solution to make the duration of a second in a day longer or shorted, because you preserve the counter value, but you no longer can rely on the fact of 1 seconds being 1000ms long! By the way the concept of leap second is nonsense to me. What is the purpose? If the purpose is to keep in sync the time with the rotation of the earth, nobody will ever notice an error of a couple of seconds, or even minutes. It makes more sense to take the current definition of a second, and then wait till the error is noticeable to an human being (let's say an error of 15 minutes, that would take centuries and we will probably far gone from the planet) and adjust it by shifting all the timezone for that amount of time.

elfchief 4y ago
Actually, UTC -is- continuous. Even with leap seconds. At no point does UTC go backwards, at no point does a second happen twice, and at no point does a second change in length -- your 'extra' second happens as 23:59:60; when there's a negative leap second, the last second of the day is 23:59:58. UTC is not expressed or defined as an offset from some time in the past. The real problem for computing is when UTC is converted to unix epoch time, which is defined as an offset from the past, and by definition has exactly 86400 seconds in a day, every day, so some provision has to be made for those extra (or missing) seconds. And -that- is where the problem happens. But it's not UTC that's mucking around with the definition of time, it's the standard representation of time that's used in modern computing that causes the problems. That being said, UTC is still probably to blame for most of the problems, because it effectively requires knowledge of more than just a timestamp to understand when something actually happened. And that extra knowledge (the map of when leap seconds have happened before) changes often and irregularly. Epoch time could totally be redefined to include leap seconds, and that would solve lots of problems, but there's no practical way to distribute that updated leap seconds table to every system that would possibly need it...
a9h74j 4y ago
> and adjust it by shifting all the timezone for that amount of time Does this follow as a question?: Shift the timezones geographically, or in terms of how they all define [true monotonic] to local-time translation?

deleted 4y ago

[deleted]

akira2501 4y ago

That would most be a local problem and would not continue to exist for a long period of time. tzdata is complex, but having waded through it a few times, it seems needlessly so for the desired use cases. There is more editorial comments than data in the files, and they spend a lot of time and effort creating entries for historical time zones so that you can convert times in the year 1942 into territorial time zones that don't even exist any more with perfect accuracy. Except, even then, it seems like it's not. If you dig into the time routines that use tzdata, you see they must use an unstable conversion process that _may_ converge, but has a simple counter protecting it in the case that it _does not_ converge and in that case it just gives you a best guess time that may still be wrong anyways. Really.. the problem I have and I suspect is the majority of cases, I have a time in Some/Zone and I want to tell someone in Another/Zone what the time would be on their clock, today, right now. The time our grandfathers would have used during the War in summer is never going to be material to me in this instance or for the majority of use cases. This isn't meant to be overly critical of tzdata just a recent annoyance I experienced when trying to find a minimal code and cross language solution to the above problem.

nightpool 4y ago
> If you dig into the time routines that use tzdata, you see they must use an unstable conversion process that _may_ converge, but has a simple counter protecting it in the case that it _does not_ converge and in that case it just gives you a best guess time that may still be wrong anyways. What does this mean? Can you explain in a little more detail, or link to somewhere I might read about this? I haven't used tzdata myself, but I understand it in broad strokes as a list of time zones, and a list of which offsets apply to which timezones at which given dates. What am I missing?
- akira2501 4y ago
  See the code for __mktime_internal in glibc, specifically the case for which it returns EOVERFLOW; but in general, the whole file has many comments worth reading. https://code.woboq.org/userspace/glibc/time/mktime.c.html https://code.woboq.org/userspace/glibc/time/mktime.c.html