[tz] Question about leap seconds

Zefram zefram at fysh.org
Wed Sep 11 17:18:38 UTC 2013

Paul Eggert wrote:
>No, typically transitions are legally specified in terms of
>local time before the transition.

Indeed, but that doesn't affect my argument.

>                                   I suppose it's theoretically
>possible that an authority currently using some sort of mean time

There is no such timezone currently, and hasn't been since 1972.  If there
were, you still run into the issue that LMT amounts to UT1+offset, not
UTC+offset, and the tz database doesn't distinguish between UT1 and UTC.
(Strict meaning of UTC here, as usual for me.)  The apparent need to
transition at onset of a leap second would depend on the sub-second
difference between UT1 and UTC that the database doesn't model.

Really, you'd only have a problem here if you're simultaneously applying
the loose interpretation of UTC ~= UT1 and the strict interpretation of
UTC with leap seconds.  Unfortunately this is a common combination in
computer timekeeping.  (Also, obliquely, subject of the philosophical
opening section of my paper to which I pointed yesterday.)

>                                                     how does
>one insert a leap second at (say) 01:59:59.9 local time?

Quite.  That's the basis of my statement that timezones to be used with
leap seconds can only use offsets of whole minutes.  (A leap second occurs
everywhere simultaneously, regardless of timezone; zic's concept of a
"rolling leap second" is bogus.)

>Ah, so the local authority says "we change our time zone at 02:30
>local time", but due to a daylight-saving transition there
>are two instants of time labeled 02:30 local time,

That kind of ambiguity is nothing to do with the leap second issues
being discussed in this thread.


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