[SLUG] Re: Time Pedantry
On Apr 8, 1:54 am, Jamie Wilkinson j...@spacepants.org wrote: I don't understand what you mean; TAI has been around since 1955. What is special about next month that we go back to 1955? TAI can be extrapolated back to 1955, but it has only existed since 1971, and for all practical purposes it was irrelevant to any time stamp until 1972. TAI is not defined for any instant until the publication of the issue of BIPM Circular T that covers that instant. Circular T is published monthly. It is not possible to assert the value of TAI now, only some approximation of TAI. Only BIPM can assert what is or is not TAI lest things get confused as people start abusing phrases such as official TAI as made distinct from something which is not TAI but which gets called TAI. http://www.bipm.org/jsp/en/TimeFtp.jsp?TypePub=publication#TAIPubFormPubli2 That exists and it's called zoneinfo and it gets updated all the time as a result of arbitrary and unpredictable human fiat. zoneinfo is for timezones, it doesn't convert between timescales. At a technical level I don't see a substantive difference. POSIX demands that zoneinfo handle offsets of seconds. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
On 8 April 2010 12:39, Steve Allen sla29...@gmail.com wrote: On Apr 7, 7:03 pm, Jamie Wilkinson j...@spacepants.org wrote: If time_t was defined as a representation of TAI instead of UTC, it'd be harder to get it wrong. Except that TAI is not available until next month and the BIPM recommends against the use of TAI in any real-time system. I don't understand what you mean; TAI has been around since 1955. What is special about next month that we go back to 1955? Will we need more zoneinfo updates? Of course then you'd need a presentation layer to convert TAI into your local human calendar, but that's pretty simple once the model and view are separated That exists and it's called zoneinfo and it gets updated all the time as a result of arbitrary and unpredictable human fiat. zoneinfo is for timezones, it doesn't convert between timescales. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
Hmmm... -Original Message- From: Steve Allen sla29...@gmail.com To: slug@slug.org.au Subject: [SLUG] Re: Time Pedantry Date: Wed, 7 Apr 2010 19:39:46 -0700 (PDT) On Apr 7, 7:03 pm, Jamie Wilkinson j...@spacepants.org wrote: If time_t was defined as a representation of TAI instead of UTC, it'd be harder to get it wrong. Except that TAI is not available until next month and the BIPM recommends against the use of TAI in any real-time system. When is next month again :? DSL -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
[SLUG] Re: Time Pedantry
On Apr 6, 7:38 pm, Robert Collins robe...@robertcollins.net wrote: http://www.opengroup.org/onlinepubs/009695399/basedefs/time.h.html The range [0,60] seconds allows for positive or negative leap seconds. The formal definition of UTC does not permit double leap seconds, so all mention of double leap seconds has been removed, and the range shortened from the former [0,61] seconds seen in previous versions of POSIX. but they still couldn't fix the Epoch Fail http://www.ucolick.org/~sla/leapsecs/epochtime.html -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
On Wed, 2010-04-07 at 11:57 +1000, Jamie Wilkinson wrote: On 3 April 2010 12:51, Nick Andrew n...@nick-andrew.net wrote: Pity that unix time_t ignores leap seconds :-) And the corollary that anyone using ntpd or other time synchronisation discipline now has a gettimeofday() that breaks the POSIX definition. The point of an operating system is to present a useful abstraction of the hardware, including the time of day clock. An abstraction which includes all the arcana of timekeeping isn't actually that useful for most applications. time_t -- despite its shortcomings -- is a fine abstraction, so much so that most operating systems designed since have stolen this abstraction rather than use a structure of Y/M/D H:M:S as done by pre-UNIX operating systems. If you do need to track the arcana then there are multiple abstractions of increasing complexity, all of which are deficient for some users, all of which are less and less useful to everyday applications. Dissing time_t because it is so simple is to miss the point. It is a good abstraction because it is simple and the majority of the applications which use it wouldn't know or care what to do with a leap second. As for the specific point, there's nothing to stop difftime() applying leap second adjustments. -- Glen Turner www.gdt.id.au/~gdt -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
On 8 April 2010 10:56, Glen Turner g...@gdt.id.au wrote: On Wed, 2010-04-07 at 11:57 +1000, Jamie Wilkinson wrote: On 3 April 2010 12:51, Nick Andrew n...@nick-andrew.net wrote: Pity that unix time_t ignores leap seconds :-) And the corollary that anyone using ntpd or other time synchronisation discipline now has a gettimeofday() that breaks the POSIX definition. The point of an operating system is to present a useful abstraction of the hardware, including the time of day clock. An abstraction which includes all the arcana of timekeeping isn't actually that useful for most applications. time_t -- despite its shortcomings -- is a fine abstraction, so much so that most operating systems designed since have stolen this abstraction rather than use a structure of Y/M/D H:M:S as done by pre-UNIX operating systems. If you do need to track the arcana then there are multiple abstractions of increasing complexity, all of which are deficient for some users, all of which are less and less useful to everyday applications. Dissing time_t because it is so simple is to miss the point. It is a good abstraction because it is simple and the majority of the applications which use it wouldn't know or care what to do with a leap second. The problem I've seen in the wild is that time_t looks good enough most of the time, but the arcana starts to matter in certain applications at fine resolutions, when it gets stepped during leapseconds (breaking monotonicity). People using gettimeofday to measure timing in their applications because it looks right get surprised when time goes backwards for a moment and their logs are wrong, programs crash, etc; when they really wanted to use clock_gettime(CLOCK_MONOTONIC, ...) instead but just didn't know it; the introduction of the article Daniel Pittman posted earlier (http://naggum.no/lugm-time.html) sums the problem up as the impedance mismatch between the mess that we call time, and the preciseness of time as an orderly concept in science and a mess in the rest of human existence As an aside, the point I was making before about the POSIX definition is that time_t is supposed to represent the number of seconds since the start of the Unix epoch: midnight UTC of January 1, 1970; however every time a leap second occurs, that timescale is reset. It's neither a monotonic clock nor a representation of UTC, but it frequently gets confused for both (which isn't so bad, it looks like UTC for the most part until we need to count a 61st second, but there's out human calendaring bias appearing again). Anything that assumes monotonicity and stability in the calendar is at fault; but for a specific example around time_t, any library using gettimeofday to figure out what now is, and then offering to calculate time offsets at seconds resolution over intervals of years needs to be aware of that fact; no library I'm aware of does this. Nobody cares that they celebrated 1234567890 24 seconds too late. (Hopefully nobody cares about celebrating 1234567890? ;-) People tend not to care about that resolution at that interval; too much data, so what does it matter anyway? (Aside aside: the perl calendaring libraries do make an effort to do the right thing if you need to know the number of days between now and before the Gregorian calendar started, which is roughly the same concept on a coarser granularity of phase.) If time_t was defined as a representation of TAI instead of UTC, it'd be harder to get it wrong. They're a good match; one's just a counter and the other is a stable monotonic timescale. Of course then you'd need a presentation layer to convert TAI into your local human calendar, but that's pretty simple once the model and view are separated; TAI is very easy to define, the conversion from TAI to UTC is welldefined, measuring time intervals from pairs of TAI dates is simple arithmetic because of the guarantees of that timescale. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
[SLUG] Re: Time Pedantry
On Apr 7, 7:03 pm, Jamie Wilkinson j...@spacepants.org wrote: If time_t was defined as a representation of TAI instead of UTC, it'd be harder to get it wrong. Except that TAI is not available until next month and the BIPM recommends against the use of TAI in any real-time system. Of course then you'd need a presentation layer to convert TAI into your local human calendar, but that's pretty simple once the model and view are separated That exists and it's called zoneinfo and it gets updated all the time as a result of arbitrary and unpredictable human fiat. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
On 3 April 2010 12:51, Nick Andrew n...@nick-andrew.net wrote: On Fri, Apr 02, 2010 at 01:33:03PM +1100, Jake Anderson wrote: We should all just use unix timestamp for all date/time communications and be done with it. Pity that unix time_t ignores leap seconds :-) And the corollary that anyone using ntpd or other time synchronisation discipline now has a gettimeofday() that breaks the POSIX definition. (I actually love that this thread came up. Time is full of trivia and holes and arcanity!) -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
[SLUG] Re: Time Pedantry
Jamie Wilkinson j...@spacepants.org writes: On 1 April 2010 16:56, Daniel Pittman dan...@rimspace.net wrote: Nick Andrew n...@nick-andrew.net writes: On Thu, Apr 01, 2010 at 03:39:00PM +1100, Daniel Pittman wrote: If it was my call, I would probably do the same thing. Way too many developers get simple things like this day has no 2:30AM or this day has two 2:00AMs wrong. That's why Daylight Savings is fundamentally evil. Too much time data is stored in non-canonical formats. ...but the real question is if we love or hate the GMT/UTC difference, and 23:59:61? *cough* :60 *cough* Well, I am glad someone was on the ball enough to notice that. ;) IIRC, :61 is actually a possible but extremely unlikely time value, to account for two leap-second adjustments required in a year, but a quick look around suggests that memory was wrong. So, :60 it is. [...] (And, finally, for anyone who really wants to despair at the whole thing, I give you The Long, Painful History of Time, which is the best write-up I know of about the engineering difficulties of the topic: http://naggum.no/lugm-time.html ) I for one am glad such pages exist. I wish the inventors of time_t had read it. I wish that an awful lot of people had spent an awful lot more time looking at what other systems around them were doing, so that I didn't have this terrible feeling that we are finally dragging our system up to the 1980s for the second or third time. Ah, well. Daniel -- ✣ Daniel Pittman✉ dan...@rimspace.net☎ +61 401 155 707 ♽ made with 100 percent post-consumer electrons -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
On Wed, 2010-04-07 at 12:27 +1000, Daniel Pittman wrote: ...but the real question is if we love or hate the GMT/UTC difference, and 23:59:61? *cough* :60 *cough* Well, I am glad someone was on the ball enough to notice that. ;) IIRC, :61 is actually a possible but extremely unlikely time value, to account for two leap-second adjustments required in a year, but a quick look around suggests that memory was wrong. So, :60 it is. http://www.opengroup.org/onlinepubs/009695399/basedefs/time.h.html The range [0,60] seconds allows for positive or negative leap seconds. The formal definition of UTC does not permit double leap seconds, so all mention of double leap seconds has been removed, and the range shortened from the former [0,61] seconds seen in previous versions of POSIX. -Rob signature.asc Description: This is a digitally signed message part -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
Robert Collins robe...@robertcollins.net writes: On Wed, 2010-04-07 at 12:27 +1000, Daniel Pittman wrote: ...but the real question is if we love or hate the GMT/UTC difference, and 23:59:61? *cough* :60 *cough* Well, I am glad someone was on the ball enough to notice that. ;) IIRC, :61 is actually a possible but extremely unlikely time value, to account for two leap-second adjustments required in a year, but a quick look around suggests that memory was wrong. So, :60 it is. http://www.opengroup.org/onlinepubs/009695399/basedefs/time.h.html The range [0,60] seconds allows for positive or negative leap seconds. The formal definition of UTC does not permit double leap seconds, so all mention of double leap seconds has been removed, and the range shortened from the former [0,61] seconds seen in previous versions of POSIX. Oh, /that/ was where it came from. Thanks. :) Daniel -- ✣ Daniel Pittman✉ dan...@rimspace.net☎ +61 401 155 707 ♽ made with 100 percent post-consumer electrons -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
Indeed. The Earth's rotational period does vary slightly (effect of earthquakes notwithstanding). One reason time is hard to deal with sensibly is our insistence on synchronising it to the mean solar day. 'Zactly! This time issue is easy to solve. ALL computers should be on UTC. All transactions should be stored in UTC. Pilots have done it this way for years. When you get a weather forecast the times on it are stated in Zulu. (UTC). Then it doesnt' matter if you are taking off in Perth and flying to Sydney, or Darwin or if you are flying at the time Daylight savings changes over, Or indeed if you are leaving a country / state where daylight savings is changing and going to one that doesn't (e.g travelling from NSW into QLD at 15:30 UTC 3rd April). The time that the forecast was issued and the time period for which it is valid are unambiguous. Local time is simply a presentation layer issue. What time is it? It's 15:30 Zulu. It's up to the presentation layer to then ask the user what timezone they are in (or would like to see the value presented in) and to then convert the UTC value to the required timezone. The fact that a Major National bank of a nation that spans a number of time zones, should store it's transactions in NSW local time zone is a indictment of poor design. Switching to using Linux wouldn't solve this problem, it's a symptom of poor system design. Pete -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
On Fri, Apr 02, 2010 at 01:33:03PM +1100, Jake Anderson wrote: We should all just use unix timestamp for all date/time communications and be done with it. Pity that unix time_t ignores leap seconds :-) Nick. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
[SLUG] Re: Time Pedantry
Peter Hardy pe...@hardy.dropbear.id.au writes: On Thu, 2010-04-01 at 16:56 +1100, Daniel Pittman wrote: ...but the real question is if we love or hate the GMT/UTC difference, and 23:59:61? Also, do we hate the earthquake that changed the length of the day for messing with our time-keeping? http://www.sciencedaily.com/releases/2010/03/100302084522.htm [...] None of this would be a problem if we'd just switch to decimal time in a single timezone and call it a day. Actually, we would still have to deal with the changing length of the day, and with the increasing difference between our stellar and earth-based times. :) Daniel -- ✣ Daniel Pittman✉ dan...@rimspace.net☎ +61 401 155 707 ♽ made with 100 percent post-consumer electrons -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
On Thu, Apr 01, 2010 at 06:25:46PM +1100, Daniel Pittman wrote: None of this would be a problem if we'd just switch to decimal time in a single timezone and call it a day. Actually, we would still have to deal with the changing length of the day, and with the increasing difference between our stellar and earth-based times. :) Indeed. The Earth's rotational period does vary slightly (effect of earthquakes notwithstanding). One reason time is hard to deal with sensibly is our insistence on synchronising it to the mean solar day. Nick. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
Nick Andrew wrote: Indeed. The Earth's rotational period does vary slightly (effect of earthquakes notwithstanding). One reason time is hard to deal with sensibly is our insistence on synchronising it to the mean solar day. // off topic Easter Time time ramblings Isaac Asimov figured it out years ago. From memory ... Create a new calendar with 52 weeks of 7 days = 364 days. Add one extra day, called World Day, at the end - 365 days. World Day does not have a day of the week. In this way, every date falls on the same day of the week in every year. For leaps years, add an extra Leap day after World Day. It too has no day of the week. To make things precise, every 100 years, there is no Leap Day, but every 400 years there is. That pretty well matches up the solar year to the earth's rotation. Easter Sunday would still be a lunar-based nightmare. Either that or redefine it to fall on the same date always, or perhaps just fall away completely. I don't recall Asimov dealing with the tetchy problem of daylight time. As for rejigging the months? I leave that as as exercise. cheers rickw -- _ Rick Welykochy || Praxis Services Hofstadter's Law. It always takes longer than you expect, even when you take into account Hofstadter's law. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
Rick Welykochy r...@praxis.com.au writes: Nick Andrew wrote: Indeed. The Earth's rotational period does vary slightly (effect of earthquakes notwithstanding). One reason time is hard to deal with sensibly is our insistence on synchronising it to the mean solar day. // off topic Easter Time time ramblings Isaac Asimov figured it out years ago. From memory ... Create a new calendar with 52 weeks of 7 days = 364 days. Add one extra day, called World Day, at the end - 365 days. World Day does not have a day of the week. In this way, every date falls on the same day of the week in every year. Note that this doesn't address the GMT/UT[C01] issue, which is all about the relationship between local time and time-as-seen-by-astronomers, or perhaps more clearly, time as it relates to the actions of things other than our own planet rotating. For leaps years, add an extra Leap day after World Day. It too has no day of the week. To make things precise, every 100 years, there is no Leap Day, but every 400 years there is. That pretty well matches up the solar year to the earth's rotation. I don't find this convincing, FWIW, since it doesn't address issues like every five days in a terribly meaningful day. All it does is translate those into one of two problems: Either you have every five days, except once a year when it is six or seven days between instances, or every five days, but which day changes every year. Unfortunately, we can't just stop the world for world day, which means that we still have unpredictable day/date matching. [...] I don't recall Asimov dealing with the tetchy problem of daylight time. IIRC he thought it was a silly idea, as were the politically motivated time zones. Both views are ... arguably true. :) Daniel -- ✣ Daniel Pittman✉ dan...@rimspace.net☎ +61 401 155 707 ♽ made with 100 percent post-consumer electrons -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
We should all just use unix timestamp for all date/time communications and be done with it. There I fixed it, http://thereifixedit.files.wordpress.com/2010/03/129138460976317329.jpg Hail me as leader Daniel Pittman wrote: Rick Welykochy r...@praxis.com.au writes: Nick Andrew wrote: Indeed. The Earth's rotational period does vary slightly (effect of earthquakes notwithstanding). One reason time is hard to deal with sensibly is our insistence on synchronising it to the mean solar day. // off topic Easter Time time ramblings Isaac Asimov figured it out years ago. From memory ... Create a new calendar with 52 weeks of 7 days = 364 days. Add one extra day, called World Day, at the end - 365 days. World Day does not have a day of the week. In this way, every date falls on the same day of the week in every year. Note that this doesn't address the GMT/UT[C01] issue, which is all about the relationship between local time and time-as-seen-by-astronomers, or perhaps more clearly, time as it relates to the actions of things other than our own planet rotating. For leaps years, add an extra Leap day after World Day. It too has no day of the week. To make things precise, every 100 years, there is no Leap Day, but every 400 years there is. That pretty well matches up the solar year to the earth's rotation. I don't find this convincing, FWIW, since it doesn't address issues like every five days in a terribly meaningful day. All it does is translate those into one of two problems: Either you have every five days, except once a year when it is six or seven days between instances, or every five days, but which day changes every year. Unfortunately, we can't just stop the world for world day, which means that we still have unpredictable day/date matching. [...] I don't recall Asimov dealing with the tetchy problem of daylight time. IIRC he thought it was a silly idea, as were the politically motivated time zones. Both views are ... arguably true. :) Daniel -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
Re: [SLUG] Re: Time Pedantry
Jake Anderson wrote: We should all just use unix timestamp for all date/time communications and be done with it. There I fixed it, http://thereifixedit.files.wordpress.com/2010/03/129138460976317329.jpg Hail me as leader Hey Leader, Are the horses drawing a UTC cart clock or sumthink? Happy Yeaster on 1270187886. Is the next SLUG meeting at 1273927886? CYA on 1271187886. cheers rickw -- _ Rick Welykochy || Praxis Services Hofstadter's Law. It always takes longer than you expect, even when you take into account Hofstadter's law. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
