[ntp:questions] Purpose of a leap second file?

Dave Hart hart at ntp.org
Thu Feb 16 13:43:08 UTC 2012

On Thu, Feb 16, 2012 at 07:25, David J Taylor
<david-taylor at blueyonder.co.uk.invalid> wrote:
> Ah, I found the instructions about using the file:
>  http://support.ntp.org/bin/view/Support/ConfiguringNTP#Section_6.14.

I've updated that section to give a few more details.

> I had the impression, falsely, from the way people were talking that it was
> an OS thing, and not an NTP thing.

It's handled by both ntpd and the OS in the common situation of a
kernel with precision timekeeping extensions.  ntpd notifies the
kernel loop discipline which takes care of implementing the insertion.

> Still don't know if Windows supports it.

Windows doesn't have those extensions.  When the kernel loop
discipline is unavailable or disabled, the daemon loop discipline
implements insertion by stepping the clock backward one second on
POSIX systems.  On Windows, the clock is run at half speed for two

>  I also see that entry format is
> different for the current entry compared to all previous, so I hope the
> white-space count isn't important....

I'm not sure what you're referring to.  Here's the prior entry and the latest:

3439756800	34	# 1 Jan 2009
3550089600	35	# 1 Jul 2012

You can verify the leapsecond file is loaded by seeing tai=34 among
the output of ntpq -crv

Dave Hart

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