[LEAPSECS] Crunching Bulletin B numbers (POSIX time)

Warner Losh imp at bsdimp.com
Tue Feb 22 16:31:53 EST 2011

On 02/19/2011 15:13, Tim Shepard wrote:


>> this is why leap seconds announced ten years in advanced

>> are important: they allow for a stand-alone machine, albeit

>> one that only needs to have it's software upgraded once in

>> ten years.


> A stand alone machine is going to have its clock drift by more than a

> few seconds over 10 years. Leap seconds are in the noise compared

> to errors due to accumulated drift of standalone machines.


> If it has a way of receiving a time signal to stay synchronized, then

> it ought to have a way of receiving info about the leap seconds (if

> they even matter).

Having a known list of leap seconds let one recover TAI time from a cold
GPS receiver in a few seconds to a minute, rather than waiting ~20
minutes for the almanac to download. Many systems have their internal
clocks in TAI, but also have requirements to report time in UTC to
users. Such a table would ensure these systems could come back online
quickly without having to pay the 20 minute penalty. While the system
may have already been down a little while when the swap is made, an
extra 20 minutes can kill reliability targets for the system.

As someone that's had to have their real-time system in a time-scale
that doesn't have leap seconds, but also report timestamped events in
UTC to the user, the 10-year horizon solves many problems with leap seconds.

As to Posix, I think the standalone system is in the noise compared to
the amount of code that actively depends on leap seconds not existing.
It might make a good talking point, but if you eliminate that
requirement, you still have the dusty deck problem to cope with.


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