[ntp:questions] basic questions about the leapsecond

Greg Dowd GDowd at symmetricom.com
Mon Dec 15 20:40:08 UTC 2008


I think it is the original rfc1305 (NTPv3) language that determined that
the bits only show up on the day of the event.  While the actual
requirement is merely that they are set before 23:59 and cleared after
midnight, the supporting text is shown below.


"On the day prior to the insertion of a leap second the leap bits
(sys.leap) are set at the primary servers, presumably by manual means.
Subsequently, these bits show up at the local host and are passed to the
local-clock procedure. This causes the modulus of the time variable,
which is the length of the current day, to be increased or decreased by
one second as appropriate. Immediately following insertion the leap bits
are reset. Additional discussion on this issue can be found in Appendix
E."

>From Appendix E
"The chronometry involved can be illustrated with the help of Figure 8,
which shows the details of seconds numbering just before, during and
after the last scheduled leap insertion at 23:59:59 on 31 December 1989.
Notice the NTP leap bits are set on the day prior to insertion, as
indicated by the <169>+<170> symbols on the figure. Since this makes the
day one second longer than usual, the NTP day rollover will not occur
until the end of the first occurrence of second 800. The UTC time
conversion routines must notice the apparent time and the leap bits and
handle the timescale conversions accordingly. Immediately after the leap
insertion both timescales resume ticking the seconds as if the leap had
never happened. The chronometric correspondence between the UTC and NTP
timescales continues, but NTP has forgotten about all past leap
insertions. In NTP chronometric determination of UTC time intervals
spanning leap seconds will thus be in error, unless the exact times of
insertion are known."

When ntpv4 is standardized, the language changes to "leap at the end of
the current month".  


-----Original Message-----
From: questions-bounces+gdowd=symmetricom.com at lists.ntp.org
[mailto:questions-bounces+gdowd=symmetricom.com at lists.ntp.org] On Behalf
Of David J Taylor
Sent: Monday, December 15, 2008 9:39 AM
To: questions at lists.ntp.org
Subject: Re: [ntp:questions] basic questions about the leapsecond

David L. Mills wrote:
> Guys,
>
> See http://www.eecis.udel.edu/~mills/ntp/html/ntpd.html#leap for what
> actually is in the implementation. As visivle here, the Spectracom
> (GPS/WWVB) driver, ACTS telephone modem driver and WWV audio driver do
> correctly display leap information. The Meinberg GPS receiver, EndRun
> CDMA receiver and audio IRIG driver do not display leapsecond
> information.
[]
> Dave

Dave,

Thanks for that summary.  My own GPS system using the Garmin GPS18 LVC
and 
the official ntp code for FreeBSD also does /not/ show the leap second.

Is this something fundamental to all GPS - in that they don't indicate 
until nearer the end of the month?

Thanks,
David 

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