[tz] Fractional seconds in zic input

[Paul Eggert]

On 02/05/2018 09:27 AM, Howard Hinnant wrote:
> Let’s assume for the moment that the tzdb compiler desires to exactly represent all of the data contained in the tzdb.
Thanks, I hadn't considered the possibility that a compiler would be 
trying to track tzdata itself.

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.