[LEAPSECS] JD & MJD, UT1 & UTC

[Martin Burnicki]

John Sauter wrote:
> On Wed, 2017-01-04 at 15:22 +0100, Martin Burnicki wrote:
>> John Sauter wrote:
>>> I did some experimenting with this on Fedora 25, Linux kernel 
>>> 4.8.15- 300.fc25.x86_64.  I found that adjtimex sets STA_NANO
>>> and returns nanoseconds only when NTP has been running for a
>>> while. John Sauter (John_Sauter at systemeyescomputerstore.com)
>> 
>> Ah, interesting.
>> 
>> Did you also have a chance to see if the kernel returns
>> microseconds rather then nanoseconds if ntpd has *not* set this
>> flag?
>> 
>> Thanks, Martin
> 
> Yes.  I determined that if NTPD has been running for only a short
> time the value returned is microseconds and the STA_NANO flag is
> not set.

Ah, OK, so the kernel handles this indeed dynamically.

> The code looks like this:
> 
> /* Subroutine to return the current UTC time in a tm structure *
> and the number of nanoseconds since the start of the last second.
>  */ int time_current_tm_nano (struct tm *current_tm, int
> *nanoseconds) { struct timex current_timex; int adjtimex_result;
> 
> /* Fetch time information from the kernel.  */ current_timex.status
> = 0; current_timex.modes = 0; adjtimex_result = adjtimex
> (&current_timex);
> 
> /* Format that information into a tm structure.  */ gmtime_r
> (&current_timex.time.tv_sec, current_tm);
> 
> /* If the kernel told us we are in a leap second, increment * the
> seconds value.  This will change it from 59 to 60.  */ if
> (adjtimex_result == TIME_OOP) { current_tm->tm_sec =
> current_tm->tm_sec + 1; }
> 
> /* Return the number of nanoseconds since the start of the last *
> second.  If the kernel's clock is not being controlled by * NTP it
> will return microseconds instead of nanoseconds.  */ if
> (current_timex.status & STA_NANO) *nanoseconds =
> current_timex.time.tv_usec; else *nanoseconds =
> current_timex.time.tv_usec * 1e3;
> 
> return (0); }

Thanks. That's basically just how I would implement it.

> During the recent leap second I also verified that adjtimex returns
> TIME_OOP.

Yes, I've observed that, too. I just wasn't sure how STA_NANO is
handled by the kernel.

Martin