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

[Warner Losh]

On Tue, Jan 15, 2019 at 11:58 AM Gary E. Miller <gem at rellim.com> wrote:

> Yo Tom!
>
> On Tue, 15 Jan 2019 10:50:10 -0800
> "Tom Van Baak" <tvb at LeapSecond.com> wrote:
>
> > Nope. All minutes have 60 seconds in Excel. And in Windows. And in
> > Unix/POSIX.
>
> Not quite.  Check out "man gmtime" for one way that POSIX represents time.
>

Yes and no. time_t doesn't have leap seconds. At all. Ever. So from that
perspective, POSIX can't do leap seconds. time_t is so engrained in the
other interfaces of POSIX, they can't have leap seconds either.


> Specifically:
>
>            struct tm {
> [...]
>                int tm_sec;    /* Seconds (0-60) */
>
> gmtime() is perfectly capable of reporting 61 seconds in a minute.
>
> Failure to account for this causes mnay problems.
>
> What it does not do well is accounting for leap seconds that are in
> the past or future.
>

Except that's not a leap second. struct tm doesn't do leap seconds really.
It kinda sorta does, but it kinda sorts doesn't.

First, when you convert back and forth to time_t, you lose leap second
information. It's impossible to convert back and forth from at least the
UTC time zone. There's some 'right' time zones, but they aren't strictly
speaking POSIX compliant because it uses the wrong math. Non-conforming,
but damned useful and not wrong.

Second, many of the tm_xxxx fields can create a time that's N xxxx's in the
future by adding N to that structure and converting. So tm_sec == 60 could
be a leap second, or it could be the second after tm_sec == 59.


> > 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.
>
> But gpsd and ntpd try real hard to be correct.  Sadly, with leap smear,
> there is no consensus on what is 'correct'.
>

Both leap seconds and leap second smear must die. Others have different
opinions, some strongly held. Not all the contradictory opinions are
necessarily wrong.

Warner
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