[LEAPSECS] Negative TAI-UTC

[Brooks Harris]

On 2017-02-04 12:24 PM, Warner Losh wrote:
> On Sat, Feb 4, 2017 at 9:41 AM, Clive D.W. Feather <clive at davros.org> wrote:
>> Looking only into the future, not historical data, what do people think the
>> probability is that TAI-UTC will ever be negative? Should data structures
>> be designed to handle this case or not bother?
> Doubtful, but not impossible.
>
> LoD would have to increase significantly for that to happen. The
> current LoD delta is about 1ms. This looks to vary between -500us to
> 2ms on IERS' web page (http://hpiers.obspm.fr/eop-pc/index.php).  It
> also looks like the short term forecast is to return towards 0 in the
> next 150 days or so. Looking at even longer term data from
> http://www.usno.navy.mil/USNO/earth-orientation/eo-products/long-term
> we see huge error bars for LoD just a few hundred years ago and a big
> drop in deltaT from 1850-1900. So the data we have suggests that it is
> unlikely, but not outside the realm of possibility.
>
> It's quite likely that the first negative leap second would have a
> much higher bug rate than the current positive leap code which has at
> least been tested several times and is known to still have issues.
Yes, a negative Leap Second seems especially dangerous to me. How many 
devices and applications have been exhaustively tested for that 
condition? I'd think the IERS should be especially nervous about issuing 
a negative Leap Second. The smears would have to go the other way too.

What if the negative Leap Second possibility was eliminated from the 
specification so only positive Leap Seconds were allowed? If the 
DUT1-to-UTC <.9s were relaxed, how far from DUT1 might it go? Would it 
matter, how much, and to whom?

That would have a rather significant impact on simplification of 
implementations, eliminating one whole set of cases and test conditions, 
wouldn't it? That could lead to more reliable and uniform behavior, 
couldn't it?

-Brooks


>
> Warner
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