REMINDER: LEAP SECOND

Tue Jun 23 09:25:32 UTC 2015

Harlan,

Why should your head explode? Possibly you’re overthinking the problem.  And there is no reason (or simple way I can envision) to test my plan, as you advise, in advance. I will just block NTP in my border router temporarily. No need to make a mountain out of this molehill. Cisco, and many other NTP client gear vendors, recommend this approach, and they’ve published extensive research on the matter.

 -mel 

> On Jun 23, 2015, at 12:46 AM, Harlan Stenn <stenn at ntp.org> wrote:
> 
> This stuff can make my head explode.
> 
> When a leap second is added, like on 30 June 2015 at the last second of
> the day, POSIX insists that the day still have 86400 seconds in it.
> This makes the day longer by one second, so time has to either slow down
> or move backwards.
> 
> The "dumb" way to do this is to step the clock back by 1 second at the
> instant before the stroke of midnight.
> 
> The allegedly better way to do this would be to stop the clock a bit
> before midnight, and hold the time for 1 second.  To continue providing
> monotonic time, every time somebody says "what time is it" during that
> holding period one would want to bump the time by the smallest amount
> possible, usually 1 nanosecond (assuming the kernel keeps time in
> nanoseconds).
> 
> Ideally you wouldn't want to add enough nanoseconds to cause the clock
> to roll over into the next day "too early".
> 
> But apparently nobody has implemented this, even though Prof. Mills
> described it in RFC-i-forget about 20 years ago.
> 
> This is mostly because POSIX deals with absolute time and not relative
> time.
> 
> In the unlikely event of a leap second deletion, there would be no
> 23:59:59, so when the clock is about to strike 23:59:59 it's OK to add
> an extra second to the clock to effectively have the time "jump" from
> 23:59:58 to 00:00:00.  This is still a monotonic increment in time.
> 
> Whatever you decide to do, I recommend you actually test it out to see
> if it behaves the way you think it will.  You have a whole week still.
> 
> H