[HN Gopher] Leap Second Smear
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Leap Second Smear
Author : edward
Score : 51 points
Date : 2021-08-03 10:54 UTC (2 days ago)
(HTM) web link (developers.google.com)
(TXT) w3m dump (developers.google.com)
| [deleted]
| gmrple wrote:
| Why? Why don't we use TAI or GPS time for generic world wide
| coordinated computer time?
| dmitriid wrote:
| As others pointed out:
|
| 1. because we want monotonously increasing time on our computer
| systems.
|
| 2. because software deals with leap anything very badly
|
| 3. [not mentioned]: we are in danger of running into negative
| leap seconds very soon:
| https://twitter.com/ariadneconill/status/1422163289518313474
| nonfamous wrote:
| Interestingly, you can accommodate negative leap seconds with
| smears as well. FTA:
|
| >> A negative leap second, if one were ever to occur, would
| be smeared by speeding up clocks over the 86,399 SI seconds
| from noon to noon.
| aidenn0 wrote:
| Because we want to unambiguously refer to times in the future?
|
| [edit]
|
| It would of course have been possible to have NTP be based of
| of TAI rather than UTC (and that might have been a good idea),
| but it still begs the question of what any POSIX operating
| system will do given that posix timestamps are leap-second
| adjusted. And the reason that POSIX timestamps are leap-second
| adjusted is so that 2628198843 has always, and will always
| represent 2053-04-13T23:14:03Z. For better or worse, humans
| have standardized on UTC, so machines that expect to interact
| with humans must also do so.
| dmm wrote:
| I would argue UTC timestamps are still ambiguous, just in a
| different way.
|
| How many seconds between now and 2038-01-01T00:00:00?
|
| It's impossible to say because we can't predict future leap
| seconds.
|
| Many uses of future times are always going to be ambiguous
| because humans care about timezones, which can change at any
| time.
| aidenn0 wrote:
| That is true, but again humans.
|
| Humans will be less bothered by you being off by 10s of
| seconds for "how many seconds until X" where X is multiple
| years in the future than they will be where X is a minute
| in the future. Any timestamp created today may be used far
| in the future, but durations are (by their very nature)
| transient. Even if you were to do a sleep(nominal seconds
| until 2038) the computer is likely to reboot before the
| interval expires, mooting any issues.
| dharmab wrote:
| The shortest possible TL;DR of
| https://en.wikipedia.org/wiki/Leap_second#History appears to be
| "because TV and radio broadcast systems in the 1950s were based
| on solar time rather than astronomical time and wanted to stay
| in sync to each other."
|
| There seems to have been a movement against leap seconds:
| https://en.wikipedia.org/wiki/Leap_second#International_prop...
| gopalv wrote:
| > Why don't we use TAI or GPS time for generic world wide
| coordinated computer time?
|
| I think that is valid as long as systems are talking to
| systems, but the interface with the world (when things are
| happening in the world) is where the solar day or year is still
| relevant.
|
| I'm sort of joking, but watching Interstellar made me kinda
| cringe about what interstellar gravity wells is going to do to
| time-keeping, even if we use something like an atomic clock to
| keep time.
|
| We will slowly get better at this until we discover something
| new, but the switch doesn't mean anything until the costs
| outweigh the change.
|
| And the leap second is going look like 46-45 BCE[1].
|
| [1] - https://www.youtube.com/watch?v=fD-R35DSSZY
| repsilat wrote:
| In some polls I've seen there's plurality support for
| "permanent 'summer time'". People don't seem to care much
| that noon in civil time maps exactly to the sun being at its
| peak.
|
| Leap second drift is super slow, too -- IIRC something like
| 10 minutes per millennium. If we shift our clocks by an hour
| in the year 8000 I think that's less disruptive than leap
| seconds every year or so.
| FredPret wrote:
| Keeping our clocks exactly aligned using leap seconds
| probably makes something like astronavigation easier
| dharmab wrote:
| No, it's the other way around. Astronomical observation
| requires TAI, and leap seconds are cited as a
| navigational hazard.
| shadowgovt wrote:
| Among other reasons (and in line with why Google cares to keep
| things tied to astronomical time measurments instead of
| perfect-period atomic clock measurements): if a user schedules
| something to happen at 'noon every day', they become
| dissatisfied if the timing of the event begins to drift off of
| "sun overhead" time consistently because of leap seconds.
| dharmab wrote:
| There are very few times and places on earth where the sun is
| directly overhead at noon. For example, solar noon is at a
| different time depending on where within a time zone you
| live.
| toast0 wrote:
| Sun overhead time drifts around throughout the year anyway, a
| couple of seconds is much less of a difference than the
| difference due to seasons and much less than DST.
|
| For example, solar noon is 1:15pm today in Seattle, and will
| be 1:09pm at the end of the month. Way more variability than
| the accumulated leap seconds over a century.
| 3pt14159 wrote:
| I know. I know. It's bananas. People keep making time more and
| more complicated. Different kinds of smears, changes in the
| dates of daylight savings times, timezone shapes influenced by
| geography instead of the amount of light.
| shadowgovt wrote:
| Honestly, time _is_ complicated. Even on Earth, where we
| rarely have to address synchronization errors due to
| relativistic effects.
|
| Users care very much about time preserving ordering of events
| and synchronization of events at disparate locales. They get
| grumpy if, say, their alarms start going off at what the
| local restaurants think is 1PM instead of noon because the
| national government passed a law starting (or stopping)
| observance of Daylight Savings this year. Similarly (though
| it's a slower-rolling error), they get grumpy if their alarms
| start firing at 11:59 and 59 seconds, 11:59 and 58 seconds,
| etc. when they have them set for noon.
|
| Time is, ultimately, a human construct and software
| management of it is beholden to the need to get it right from
| the user's point of view.
| 3pt14159 wrote:
| You were downvoted unjustly. I think your opinion is mostly
| valid, but I think people undervalue simplicity because
| they underestimate knock-on effects of their decisions.
| creeble wrote:
| It depends on what you mean by "computer time".
|
| We _do_ use it on some "computers":
|
| > "The Society of Motion Picture and Television Engineers
| (SMPTE) video/audio industry standards body selected TAI for
| deriving timestamps of media.[77] IEC/IEEE 60802 (Time
| sensitive networks) specifies TAI for all operations. Grid
| automation is planning to switch to TAI for global distribution
| of events in electrical grids. Bluetooth mesh networking also
| uses TAI.[78]"
|
| (from
| https://en.wikipedia.org/wiki/Leap_second#Workarounds_for_le...
| )
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(page generated 2021-08-05 23:01 UTC)