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Daylight savings date identification over the net

Discussion in 'Embedded' started by Dimiter_Popoff, Mar 30, 2014.

  1. About every year when it comes to it I search some reasonable
    way to implement it and shortly after I give it upand go
    empty handed.
    Does anyone know how do MS and Android devices do it?
    Where do they look the date up?
    Eventually I suppose I'll do some sort of service for
    DPS devices myself but it would be
    nice if there were something similar to NTP to use.

    Dimiter_Popoff, Mar 30, 2014
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  2. What's wrong with just using the timezone database ?

    See http://www.iana.org/time-zones

    Some background information is at: http://en.wikipedia.org/wiki/Tz_database

    Simon Clubley, Mar 30, 2014
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  3. Dimiter_Popoff

    Tom Gardner Guest

    Fine, provided you know location, can interpret the
    database correctly(!) and can update the embedded
    equipment when the rules change.

    Anything to do with calendars is 10% more difficult than
    you thought. Recursively.

    Favourite questions for youngsters:
    - how many days in a year
    - how many months in a year
    - how many hours in a day
    - how many seconds in a minute
    Clueless youngsters /usually/ get one of those right;
    very few youngsters get them all right.
    Tom Gardner, Mar 30, 2014
  4. Dimiter_Popoff

    upsidedown Guest

    There is no such thing !!

    Last night Crimea went from UTC+2 (East Europe Standard Time) to
    Moscow time (UTC+4 all year around) at UTC 20:00 yesterday.

    The rest of Europe did the transition at 01:00 UTC from standard to
    daylight saving time.
    In Microsoft environment, the TZ environmental variable is quite
    expressive, but who is going to update for your region ? Mr. Putin ?
    NTP only carries the leap second warning signal. The daylight saving
    time is far too political to be handled technically :-(
    upsidedown, Mar 30, 2014
  5. Dimiter_Popoff

    Nils M Holm Guest

    I'm not exactly a youngster, but given the superficial
    simplicity of your question, I guess I am missing something
    important. Can you expand a bit on the topic or point to
    resources that explain all the nitty gritty bits?
    Nils M Holm, Mar 30, 2014
  6. Dimiter_Popoff

    Paul Rubin Guest

    Astronomy buffs will probably get them right ;-). My guesses:

    Days in a year: http://en.wikipedia.org/wiki/Year#Calendar_year
    Simplified answer: about 365.2425 days. The ancient Romans approximated
    this as 365.25 days, handled with a 365-day calendar plus 1 leap day
    added every 4 years, the so-called Julian calendar. The discrepancy
    built up over the centuries til it was adjusted in the 18th century
    (depending on where you were), by skipping 11 days and changing the leap
    year algorithm slightly, giving the Gregorian calendar now in use.

    Try the Unix command "cal 9 1752" or see:

    Months in a year: I think this is supposed to be the one everyone gets
    right, i.e. 12, but of course I'm suspicious. I've heard that the one
    thing that's really stayed constant is 7, the number of days in a week.

    Hours in a day: varies slightly, but the period of the earth's rotation
    (sidereal day) is about 23.9344699 hours (23h 56m 4.09 sec). The solar
    day is slightly longer (averages just a tiny bit over 24h) but its
    length varies with the season:

    Seconds in a minute: 60 but sometimes there is a forward or backward
    leap second. http://en.wikipedia.org/wiki/Leap_second
    Paul Rubin, Mar 30, 2014
  7. Dimiter_Popoff

    David Brown Guest

    I am not a youngster either, and I can have a shot here off the top of
    my head. My guess is that I'll have missed something.

    The number of days in a year depends on the leap year. The rules are, I

    if (y % 4) {
    return 365;
    } else if (y % 100) {
    return 366;
    } else if (y % 400) {
    return 365;
    } else if (y % 2000) {
    return 366;
    } else {
    return 365;

    There are 12 months in a year (I'm assuming the modern western calendar
    here, of course).

    There are 24 hours in a day.

    There are 60 minutes in an hour.

    There are 60 seconds in most minutes. But every now and again, there is
    a leap second, meaning that there are some minutes with 61 seconds.
    There is no fixed pattern in this - it is based on astronomical
    observations (whose fine details are chaotic), and decided on by some
    committee somewhere.
    David Brown, Mar 30, 2014
  8. Dimiter_Popoff

    upsidedown Guest

    354, 365, 366, etc. completely culture dependant.
    12 or 13 depending on culture.
    usually 24
    59, 60 or 61 depending on the leap second issue.
    You seem to be quite optimistic.
    upsidedown, Mar 30, 2014
  9. Dimiter_Popoff

    Rich Webb Guest

    Shouldn't the correct answer be "365 in most years and 366 in leap
    years," non-integral days being somewhat awkward to implement.
    Otherwise, we'd tack on not quite 6 hours to the end of the day on 31
    December. Which, actually, seems like a rather good idea except for
    the inconvenience of moving "midnight" around every year. ;-)
    Rich Webb, Mar 30, 2014
  10. Dimiter_Popoff

    Tom Gardner Guest

    That's a good example of the 10% rule for me.
    I've never come across the 354; was it due to
    the change from Julian to Gregorian calendar?

    Precisely. Most people fail to get that one right.

    23 today in the UK :)

    I didn't realise 59 was even a possibility, but it
    hasn't happened yet. I understands why the earth's
    rotation has slowed from a day being 6 hours,
    but I don't see how it could speed up!

    They usually get 366 days in a year, during my
    pause after they've said 365.
    Tom Gardner, Mar 30, 2014
  11. Dimiter_Popoff

    Tom Gardner Guest

    Trap avoided :)
    Not today :)
    AFAIK is it astronomical measurements that determine
    when a leap second needs to be inserted. In at least
    one year, two were inserted.
    Tom Gardner, Mar 30, 2014
  12. Dimiter_Popoff

    Coos Haak Guest

    Op Sun, 30 Mar 2014 15:31:49 +0100 schreef Tom Gardner:
    The Islamic year has 354 and 355 days.
    The Jewish year has 353, 354, 355, 383, 384, or 385 days.
    Coos Haak, Mar 30, 2014
  13. Apparently not, otherwise I should have managed to locate it
    by now... :)

    I suppose I'll start some manual service next time some of our
    users complains (timezone has to be set manually - NTP is
    also invoked manually but no one complains about that).
    So far we have sales in only 5 countries so maintaining
    the data manually should be manageable.
    I suppose automatic timezone change on a spectrometer
    which often acquires a spectrum over more than a day will
    introduce a lot worse problems to the users though....
    (the starting moment is recorded in UTC and the acquisition
    time is in seconds, with extensive dead time correction etc.
    attributes around it so the measurement will not be compromised
    but making all the considerations might be harder for the
    user than manually changing the timezone :D ).
    Hopefully not, he has a lot less to say about things here
    since Bulgaria is part of NATO and the EU. A lot more than
    most people think though (we have not yet been told what
    happened to the giant network of KGB agents which used to
    officially run the country prior to 1989 - so they now run
    it unofficially, which is some sort of progress I suppose....).

    Dimiter_Popoff, Mar 30, 2014
  14. Dimiter_Popoff

    upsidedown Guest

    While it was previously possible to add/remove 1 second twice a year,
    now there are 4 opportunities to do this each year. As far as I know,
    seconds have been added only once a year and the slowing down has
    recently been so slow that the time between additions have been
    several years.
    upsidedown, Mar 30, 2014
  15. Dimiter_Popoff

    Tom Gardner Guest

    After bothering to check, you are of course right.
    I suspect my error was based on remembering the
    occasions when they were added in July rather
    than January.
    Tom Gardner, Mar 30, 2014
  16. Large earthquakes.


    There are a number of very large quakes that are known to have
    shortened the day. However, the 8.9 in Japan produced a delta of only
    -1.8us and it's reasonable to expect that the rotational deltas
    produced by different quakes would be randomly distributed and thus
    would be expected largely to cancel. So a negative leap second is
    possible, but highly unlikely.

    Might happen if Earth happens to capture an asteroid - but then nobody
    would notice because they'd be too busy looking at the second moon.

    George Neuner, Mar 30, 2014
  17. The big question here is WHICH "year", "month", "day", "hour", "minute"
    and "second". (Are we UTC, UT0, UT1, or something else for the fine
    scale, and which calendar for the coarse scale, as well as dealing with
    sidereal or solar periods). If you give poorly defined questions, of
    coarse you are going to get wrong answers, and I suspect that most of
    the "wrong" answers you get, would be right under a different
    definition. (For instance, some systems do not have leap seconds, or
    need to worry about Day light savings time).
    Richard Damon, Mar 30, 2014
  18. Dimiter_Popoff

    Tom Gardner Guest

    I would regard anybody that gave that answer as
    someone that had given a good sufficient answer.

    All too often I've had to /explicitly explain/ why some
    days are 23 hours long. That really ought to be have been
    recognised by /anybody/ that's experienced at least 20
    such days!

    Anybody that calls themselves an engineer really ought
    to have heard of leap seconds. Then, if they ever need
    to, they can find out more detail.

    Months in a year? Knowing that 12 isn't the only answer
    is something that an inquiring intelligent educated
    well-rounded person ought to know.
    Tom Gardner, Mar 30, 2014
  19. There's the timezone database, and code libraries that use it quite
    successfully. People use and like that enough that some sleazebags with
    an astrological background (of all things) tried to gain a copyright
    stranglehold over it around 2011, to reap a profit.

    As a fall back, some C libraries also support a fancy domain-specific
    language that allows specifying the entire behaviour piled into the TZ

    The key problem is the same in both cases: the whole DST nuisance, just
    like timezone handling in general, is a politically dominated pile of
    utter nonsense. Whatever solution you have will therefore, by default,
    be wrong the next time you need it because in the meantime some
    politician or other decided that the system needed "improvement" (as in:
    it wasn't quite boneheaded enough yet).

    So there's really nothing to be done about it but to maintain an
    automatic update service to distribute (a suitable subset of) the latest
    timezone DB to your devices. For devices that don't already need such
    an auto-updater, setting one up just for this would be highly
    questionable. And of course, their place of work may well prohibit such
    kind of net access for security reasons.
    IMHO the only approach that makes technical sense for any embedded
    device (here meaning: any device that's not running a full-service
    general-purpose operating system with user-installable software and
    always-on network access) is to _not_even_try_. I.e. refuse to admit
    that not just "daylight saving", but rather the entire concept of
    timezones other than UTC (or NTP, or GPS --- pick your poison) even
    exist. The only likely result from trying to handle this pile of
    nonsense is frustration.

    Scientific instruments had better use the only somewhat scientific time
    scale there is: number of second since {some point in time of your choosing.
    Hans-Bernhard Bröker, Mar 30, 2014
  20. 365 or 365

    12, if we use the Gregorian calendar.

    24, as long as we stay in the same timezone. Otherwise, any whole
    15 minutes from 22 till 26; i.e. 22,22.25,22.5,22.75,23 etc

    59,60 or 61. The non-60 values are used to add/subtract leap

    That must be the 12 months per year.

    But only the days per week is perfectly safe.
    -- mrr
    Morten Reistad, Mar 30, 2014
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