[tz] [tz-announce] 2018f release of tz code and data available

[Paul Eggert]

Michael H Deckers via tz wrote:
> time-of-day values on or after 24 hours may be ambiguous due
>     to leap seconds.

Only if we decide they're ambiguous. Instead, let's resolve the ambiguity in the 
obvious and useful way.

>    The fictitious UTC time stamp  "2016-12-31T25Z" could
>    indeed be taken to mean
>        2016-12-31T00Z + 25 h = 2017-01-01T00:59:59Z or else
>        2017-01-01T00Z + 01 h = 2017-01-01T00:00:00Z.

Only the latter meaning is useful in tzdb, so let's do it that way. We went 
through this issue a while ago with "rolling" versus "stationary" leap seconds, 
only one of which is useful in practice, and in hindsight we should have 
supported only the useful variant.