[ntp:questions] basic questions about the leapsecond

Richard B. Gilbert rgilbert88 at comcast.net
Mon Dec 15 21:08:26 UTC 2008


Greg Dowd wrote:
> 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: 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 

Did you READ the message you replied to?

It seems quite clear to me!




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