Re: [Ntp] Antw: [EXT] Re: Timescales

[Magnus Danielson <[e-mail]>]

Hal,

On 2020-12-09 20:02, Hal Murray wrote:
> [e-mail] said:
>> Most computers work primarily in UTC.
> Most computers work on POSIX pseudo UTC time.
> (which is why we have this leap-second mess)

Most computers use one of two interpretations of POSIX Time.

One variant of UTC to POSIX time_t mapping was provided for a while, but
later removed from the POSIX documentation, mapping 23:59:60Z into
00:00:00, which is what you expect from (omitting date)

time_t = hours*3600 + min*60 + sec

and as this is re-interpreted as print-out time

sec = time_t % 60
min = (time_t / 60) % 60
hour = (time_t / 3600) % 24

reads out as 00:00:00, which next section also labelled 00:00:00. This
behavior is mimiced by the NTP time-scale for at NTPv3 and NTPv4 as I
recall it.

Then, for Linux the leap-second is not assigned the printed label
00:00:00 but the printed label 23:59:59, which keeps 00:00:00 unique to
the first second of the new UTC day. Thus, rather than double 00:00:00
read-outs you have double 23:59:59 read-outs. Thus, we cannot speak
about them both having UNIX or POSIX time as if it was the same, but
they are different and we need to treat them different. NTP
implementations needs to know how to handle it, and in practice it is
handled through the nanokernel interface [1]. The user applications
should use that interface too, to get the offset, but some operating
systems also support getting time in the TAI form, see CLOCK_TAI for
clock_gettime() [2].

While my manual references is to Linux, the BSD guys for sure can
illustrate the state there.

But, the lesson is that even within "UNIX type systems" you have a
diversity, but there may also be the common enough tool-set to help
applications to get time straight if we only enable the NTP process to
use the right interfaces and allow the protocol to transport all the
needed information.

I think some of the discussions of issues relates to older ways to get
time-stamps, which may no longer reflect the state of art available on a
wide selection of systems. The inclusion of these new interface is now
quite old, so it has rippled through the builds and support through
various libraries etc.

Cheers,
Magnus

[1] https://man7.org/linux/man-pages/man3/ntp_adjtime.3.html

[2] https://man7.org/linux/man-pages/man2/clock_gettime.2.html