I can't see the problem here. For instance, Japan is now using UTC+9, so the leap second take place right before 9am(00:00:00Z), yet 10am that day would still unambiguously mean 10am(01:00:00Z). I cannot see why it would be a problem for UTC after 24z. 2018-10-22 05:25, Michael H Deckers via tz <tz@iana.org> wrote:
On 2018-10-21 03:00, Steve Allen wrote:
Many examples of non-normalized dates and times exist in historical literature.
For time stamps of UTC, or of a time scale derived from UTC with a piecewise constant offset (as for civil time scales), time-of-day values on or after 24 hours may be ambiguous due to leap seconds.
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. That is probably one reason why the draft new version of ISO 8601 proposes to drop even the notation for the time of day 24 h after midnight (does "2016-12-31T24Z" mean 2016-12-31T23:59:60Z or 2017-01-01T00:00:00Z?).
It is true that this ambiguity can arise at most for two (or twelve) dates in a year, but a parser for zic input has to deal with all cases, and cannot be written correctly unless the notation is defined unambiguously.
There is no such ambiguity with day of the month numbers less than 01 or > ultimo, so that it is always possible to avoid any time-of-day values below 00 h or on or after 24 h in time stamps of UTC or of civil times.
Michael Deckers.