[time-nuts] Re: Another leap second problem

[Tom Van Baak]

Hi Steven,

The GPS spec is fine, although leap second calculations are a bit 
tricky. Almost all GPS receivers get it right but there's a bug in some 
that's triggered when there have been no leap seconds for more than 256 
weeks, or in this case, 512 weeks. Details at [1].

The most recent leap second was at the end of 2016-12-31, which was in 
GPS week 1877 (GPS weeks count from 1980-1-6). The bogus date you quote, 
2025-10-25, is in GPS week 2389. Note that 2389 - 1877 = 512. So yes 
this is another instance of the same receiver f/w bug.

Let us know when they identify which make/model receiver it is, and what 
protocol it is speaking.

But while I have your attention, it's also a bug in NTP. The definition 
of UTC says if a leap second occurs it will be the last day of the 
month, and the date you quote, 2026-10-25, isn't the last day. So NTP 
appears to be missing basic range checking. That you can easily fix if 
you know who maintains NTP or NTP-like software.

A cheap GPS simulator would be nice, but in this case you can simply 
manipulate the serial protocol stream to test how NTP reacts to bogus data.

/tvb

[1] http://leapsecond.com/notes/leapsec256.htm


On 11/7/2025 4:47 PM, Steven Sommars via time-nuts wrote:
> Recently a popular NTP/GNSS server began displaying an upcoming leap second
> notice  <https://community.ntppool.org/t/leap-second-in-october-2026/4132/8>
> .
> That date is slightly less than 351 days in the future.  If my math is
> correct,that corresponds to Sunday, 2026-10-25 00:00:00.
> The NTP server support team has reproduced the problem and is investigating.
>
> There was a leap seconds incident
> <https://community.ntppool.org/t/leap-indicator-set-beginning-2021-11-27-00-00/2253>in
> 2021. Look at these dates:
>
> 1792886400 2026-10-25 00:00:00  Date advertised by NTP server.   (first
> column is seconds since Unix Epoch)
>
> 1638057600 2021-11-28 00:00:00   Another leap seconds incident
> <https://community.ntppool.org/t/leap-indicator-set-beginning-2021-11-27-00-00/2253>
>
>
> 1483228800. 2017-01-01 00:00:00  Previous leap second
>
> These dates are each separated by 256-weeks.    There is a description of a
> 256-week bug at http://www.leapsecond.com/notes/leapsec256.htm
>
> How widespread is this?   Commercial grade GPS simulators are expensive.
> Could one build an SDR-based simulator on the cheap to test for such
> problems?
>
> Steve Sommars