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

[Rob Seaman]

Isn't this covered (for instance) in section 20.3.3.5.2.4(b) of
IS-GPS-200J (https://www.gps.gov/technical/icwg/IS-GPS-200J.pdf)? The
various libraries and operating systems listed deal with none of the
reference time scales (GPS, UTC, or TAI, for that matter) completely and
correctly.

There's no obvious connection here with astronomical use cases. We rely
on our reference clocks' vendor to have implemented IS-GPS-200J and
relevant standards correctly. These devices operate off single board
linux cards, but I presume they wrote the critical code themselves to
handle leap seconds, but also other precision timekeeping issues not
entertained by POSIX and other mass market software. Our clocks handle
time scales other than UTC, but since our customer (NASA PDCO) specifies
UTC, that's what we deliver.

The bread-and-butter leap second discussions on this list seem
irrelevant to simply meeting an engineering requirement to adhere to
whatever standard. If POSIX, python, excel, whatever don't provide
canned tools to convert seamlessly between GPS and UTC, or to handle
format conversions from week:ms to sexagesimal, such tools must be
implemented outside those languages/libraries. There are a vast number
of other science-related data engineering details that POSIX (for
example) also does not implement.

Rob Seaman, Lunar and Planetary Laboratory

--

On 1/15/19 11:50 AM, Tom Van Baak wrote:
> Jim -- I'm replying via the LEAPSECS list because we love leap second questions here.
>
> List -- Jim is a long-time member of time-nuts, works at JPL, and does wonderful stuff.
>
> From Jim Lux:
>> 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.
> Yes, that's one way. But that doesn't sound safe or correct to me.
>
>> 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.
> Nope. All minutes have 60 seconds in Excel. And in Windows. And in Unix/POSIX. Really any computer system that uses a fixed "epoch" and a ticking "counter" is ruined by leap seconds. The systems differ in their epoch's, they all differ in their counters, they can all be set to UTC, except they all fail when there's a positive or negative leap second.
>
>> 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.
> This is a good question for LEAPSECS. I suspect those packages are well-known here.
>
>> (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)
> Right, this is typical for almost any software developed anywhere. Leap seconds are such a weird exception, almost no commercial software, web frontend or backend, or mobile phones, or mobile apps deal with them correctly.
>
> I'm hoping a number of LEAPSECS members can chime in. There are astronomers here who must be used to solving issues like this, in python or other tools, so their input would be valuable.
>
> /tvb
>
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