[time-nuts] leapseconds, converting between GPS time (week, second) and UTC

[Bob kb8tq]

Hi

Welcome to yet another chapter in the very long book “why leap seconds are bad”. 
There are a lot of posts going into may of the ways they messes up code and updates. 
Keeping a library up to date with a leap second that might be a few months out …. yikes …..
Recompiling all your code (if it’s all compiled)  in time to incorporate the update …. even 
more “interesting”. Trusting your code to wander out on the internet to grab live information ….
I can’t think of *any* way that could go wrong …. 

Bob


> On Jan 15, 2019, at 3:33 PM, Fiorenzo Cattaneo <fio at cattaneo.us> wrote:
> 
> There are few libraries which offer time unadjusted for leap seconds,
> like TAI time:
> https://en.cppreference.com/w/cpp/chrono/tai_clock
> 
> Some other libraries (typically in written in languages which very few
> people use like haskell ) offer leapsecond support, but the
> fundamental problem (as mentioned in the writeup) POSIX requires a day
> length to be 86400 *AND* requires to be UTC. These two requirements
> are fundamentally at odds with each other, which is why it's so
> complicated to deal with this in general purpose code.
> 
> 
> 
> 
> -- Fio Cattaneo
> 
> Universal AC, can Entropy be reversed? -- "THERE IS AS YET
> INSUFFICIENT DATA FOR A MEANINGFUL ANSWER."
> 
> On Tue, Jan 15, 2019 at 12:25 PM Fiorenzo Cattaneo <fio at cattaneo.us> wrote:
>> 
>> I double check the Python code, and I can confirm it does not take
>> LEAP seconds into account. I highly doubt you will find standard time
>> libraries for the most common languages which will deal with LEAP
>> seconds. They would rather just ignore it and have one less of a
>> headache to worry about (remember all the bugs that pop up even when
>> we switch in and out of DST, like applications crashing because NTP
>> applies the 1 hour change in a discontinous manner, as well as iphone
>> alarms not working when the DST date is modified?).
>> 
>> The C++ boost time library docs seem to imply it's possible to use
>> them and have them account for LEAP seconds:
>> https://www.boost.org/doc/libs/1_58_0/doc/html/date_time.html
>> 
>> What is worse is tha the POSIX standard requires a day to be 86400
>> seconds, and pretty much all the code which relies on POSIX makes such
>> assumption, in fact in my experience the day length is usually
>> hardcoded to 86400 seconds:
>> https://www.ucolick.org/~sla/leapsecs/right+gps.html
>> 
>> =================================
>> 
>> 
>> fcattane at linux-mint-64:~$ python
>> Python 2.7.15rc1 (default, Apr 15 2018, 21:51:34)
>> [GCC 7.3.0] on linux2
>> Type "help", "copyright", "credits" or "license" for more information.
>>>>> import time
>>>>> import datetime
>>>>> 
>>>>> # last leap second got added on 1 Jan 2017
>> ... #
>> ...
>>>>> unix_time_2017_01_01 = 1483228800
>>>>> 
>>>>> 
>>>>> 
>>>>> unix_time_2017_01_01
>> 1483228800
>>>>> 
>> 
>> 
>>>>> time.gmtime(unix_time_2017_01_01 - 2)
>> time.struct_time(tm_year=2016, tm_mon=12, tm_mday=31, tm_hour=23,
>> tm_min=59, tm_sec=58, tm_wday=5, tm_yday=366, tm_isdst=0)
>> 
>>>>> time.gmtime(unix_time_2017_01_01 - 1)
>> time.struct_time(tm_year=2016, tm_mon=12, tm_mday=31, tm_hour=23,
>> tm_min=59, tm_sec=59, tm_wday=5, tm_yday=366, tm_isdst=0)
>> 
>>>>> time.gmtime(unix_time_2017_01_01)
>> time.struct_time(tm_year=2017, tm_mon=1, tm_mday=1, tm_hour=0,
>> tm_min=0, tm_sec=0, tm_wday=6, tm_yday=1, tm_isdst=0)
>> 
>>>>> time.gmtime(unix_time_2017_01_01 + 1)
>> time.struct_time(tm_year=2017, tm_mon=1, tm_mday=1, tm_hour=0,
>> tm_min=0, tm_sec=1, tm_wday=6, tm_yday=1, tm_isdst=0)
>> 
>>>>> time.gmtime(unix_time_2017_01_01 + 2)
>> time.struct_time(tm_year=2017, tm_mon=1, tm_mday=1, tm_hour=0,
>> tm_min=0, tm_sec=2, tm_wday=6, tm_yday=1, tm_isdst=0)
>> 
>> 
>> 
>> 
>> 
>> -- Fio Cattaneo
>> 
>> Universal AC, can Entropy be reversed? -- "THERE IS AS YET
>> INSUFFICIENT DATA FOR A MEANINGFUL ANSWER."
>> 
>> On Tue, Jan 15, 2019 at 10:01 AM jimlux <jimlux at earthlink.net> wrote:
>>> 
>>> I'm working with a variety of things which work in UTC or GPS
>>> week/millisecond, so we're doing a lot of conversion back and forth.
>>> (the spacecraft puts out time in week:millisecond, all the ground
>>> systems processing is in UTC)
>>> 
>>> The question comes up when converting back and forth, and whether
>>> various libraries handle leap seconds correctly.
>>> For now, I can use a hack of not computing back to 6 Jan 1980, but use
>>> an epoch like 15 Dec 2018 (week 2031: 518,400.000 seconds) and hope
>>> there's no leap second in the offing.
>>> 
>>> 
>>> For instance, in Python, if I do a datetime(2016,12,31,0,0,0) +
>>> timedelta(hours=30) does it come out as 1/1/2017 6:00:00 or 5:59:59  (it
>>> comes out 0600)
>>> 
>>> Similarly, does Excel's time formatting allow for some minutes having an
>>> extra second, or does it just assume all minutes are 60 seconds.
>>> 
>>> I'll probably test it for the cases I'm interested in (Ruby, Python,
>>> Excel, Matlab, Octave), but if someone else has already done it, then
>>> I've got something to cross check against.
>>> 
>>> 
>>> (python does NOT know about leap seconds)
>>> 
>>> import datetime
>>> 
>>> d = datetime.datetime(2016,12,31)
>>> 
>>> dt = datetime.timedelta(hours=30)
>>> 
>>> d
>>> Out[4]: datetime.datetime(2016, 12, 31, 0, 0)
>>> 
>>> dt
>>> Out[5]: datetime.timedelta(1, 21600)
>>> 
>>> d+dt
>>> Out[6]: datetime.datetime(2017, 1, 1, 6, 0)
>>> 
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