Warner Losh imp at bsdimp.com
Sat Feb 4 12:30:23 EST 2017

On Sat, Feb 4, 2017 at 10:24 AM, Warner Losh <imp at bsdimp.com> 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

Bah, of course I mean decrease.

> 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.

I should have added a "Yes, make all the things signed." I'd go so far
as to say make them 32-bits, but even 16 bits would be enough for
thousands of years into the future. It would cover thousands of years
into the past even at the most extreme points of the error bars.


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