[tz] [PROPOSED 3/3] Vanguard form now uses subsecond precision

[Paul Eggert]

On 7/28/22 10:40, Steve Allen via tz wrote:
> I urge that tz never attempt to encode zone offsets with precision
> better than 1 millisecond.

There's one instance of that in the patch, the "7:06:30.133333333" that 
comes from calculating 104° 17′ 17″ east of Paris Mean Time (00:09:21). 
The law in question gave a precision of 1 arc second for longitude, 
which works out to a precision of 66⅔ ms for UT offset.

I used nine digits because POSIX timestamps have nanosecond precision, 
and so if tzcode ever gets support for non-integer UT offsets the 
closest POSIX approximation to legal time would need the nine digits. 
(POSIX timestamps do not indicate their precision, unfortunately.)

There's a tension here between trying to make a potential POSIX 
implementation as close as possible, vs following the usual scientific 
practice where the number of digits indicates the precision and where 
"7:06:30.133333333" should presumably be replaced by "7:06:30.13" to 
reflect the 66⅔ ms precision. (Or should the last "3" be removed too? 
I'm a bit fuzzy on how best to express fuzziness when "X±Y" notation is 
not available....)