[tz] Fractional seconds in zic input

[Paul Eggert]

On 02/05/2018 10:21 AM, Howard Hinnant wrote:
> On Feb 5, 2018, at 1:01 PM, Paul Eggert <eggert at cs.ucla.edu> wrote:  >> >> In that case, how about if we follow POSIX's lead and specify 
nanosecond resolution as the highest the format supports? Although 
that's likely overkill, it does match a widely used standard; and better 
overkill than underkill. > > On Feb 4, 2018, at 7:21 PM, Howard Hinnant 
<howard.hinnant at gmail.com> wrote: >> >> In choosing a finest supported 
precision, I would encourage the choice of something coarser than 
nanoseconds.
Suppose an old UT offset uses sexagesimal notation, or something derived 
from it? In that case, the exact offset might not be representable as a 
decimal number, and the nanoseconds resolution will provide a 
comfortable excess of precision. Sexagesimal is not entirely 
hypothetical, as we have good evidence that civil time in Vietnam from 
1906 to 1911 was 104° 17′ 17″ east of Paris.

I guess I'm not seeing the harm to go with nanoseconds in the data 
format; if a downstream user wants less precision they can easily round. 
And following Steve Allen's lead, we can mention in the documentation 
that there's no practical use of sub-millisecond precision in these old 
timestamps.