[time-nuts] Examples of leap-seconds in local timezones

[Wojciech Owczarek]

Magnus,

I have little experience with radio-based public time dissemination
services, but some with GPS/NTP/PTP, so here's some info - hope it's of
some value to you.

For US and European exchanges, the leap second time happens outside trading
hours, so maybe that's why we've heard little (horror) stories. This is
definitely not an easy thing to deal with during trading, unless your
messaging protocol actually has specific leap second extensions - which I'm
not aware of any of the ones I know having.

This is how the Linux kernel running UTC does it (assuming it's been told
by NTP/PTP/GPS/user and the respective leap second kernel flag was set -
and for example, a broken IRIG-B box somewhere hadn't forgotten to tell
some of your NTP servers about this and your quorum blocked it):

- If we're removing a second, Linux system clock will change from
23:59:58.999999999 straight to 00:00:00.000000000
- If we're inserting a second, Linux system clock will run the last second
second twice: 23:59:59.999999999 will change again to 23:59:59.000000000

(that's the kernel internal / UTC time, time formatting functions will
output the :60 value at insertion time).

This approach can potentially be problematic for some applications,
especially the insertion, which is essentially a step backwards, so some
institutions / vendors propose an alternative approach, which is the leap
second smear - I think Google was advocating that one: this is where you
gradually add/take the extra time throughout the whole leap second day
(which would be an approx. 11.6 ppm offset if you started from midnight -
so you'd have to model this carefully to bring it back to normal soon after
the leap second midnight).

Those alternative methods of time insertion may be fine if they're used
only internally within an organisation that doesn't provide timestamped
data to other organisations, without also providing time services to them -
or basically, all is well as long as interconnected parties use the same
method of dealing with this.

*personal opinion*

While the leap second is a somewhat inconvenient phenomenon, while it's
still there, it's there and we have to deal with it. I think that most of
the problems around it that people talk about are a little bit of FUD
resulting purely from the lack of adequate testing. This is based on my
experience with computer/network kit - this wasn't meant to be an absolute
statement.

I'd say let the IERS keep computing it but let's drop it from UTC and let's
do a one-off leap hour in some 4,000 years :-)

Regards
Wojciech