Leap Second

[Mark Felder]

On Wed, Jul 1, 2015, at 08:47, Dag-Erling Smørgrav wrote:
> Mark Felder <feld at FreeBSD.org> writes:
> > I'm not an expert on the leapsecond operation, but if I understand it
> > correctly there are two ways a system can be notified of a leapsecond:
> > via a tzdata update or through NTP.
> 
> Answering a bit late, but no: in practical terms, only NTP works.

Better late than never :-)

> Recording leap seconds in tzdata breaks POSIX and a lot of assumptions
> in existing code, not only on the day a leap second occurs but at any
> time in history after at least one leap second has occurred.
> 

Yeah, I think it's pretty obvious now that doing leapseconds in tzdata
is a bad idea -- worse than leapseconds themselves maybe? :-)

> > 1) FreeBSD server unaware of leapsecond due to no tzdata entry and not
> > synced to NTP ends up 1 second off
> 
> A server which is not synchronized with a reliable external source will
> end up a lot more than one second off regardless of leap seconds

> because it relies solely on onboard RTCs and oscillators which are both
> inaccurate and imprecise.  Clock drift will be measured in seconds per
> week and vary depending on CPU load, disk I/O, the phase of the moon and
> your dog's horoscope.
> 

I was ignoring that bit, but it's worth pointing out to the readers. I
should have worded it "...will be one *more* second off" :-)