[tz] [PROPOSED] Fix ambiguous leapsecs by rolling up to a minute

Paul Eggert eggert at cs.ucla.edu
Thu Sep 16 16:02:52 UTC 2021

On 9/16/21 3:47 AM, Michael H Deckers wrote:

>     The charter of tzdb is to provide software
>     that represents local times as defined by local authorities.

That is indeed our goal. However, the situation in question has never 
occurred so there are no local authorities to consult. So we're trying 
to predict what future local authorities will do (a common problem in 
tzdb, unfortunately...).

My guess is that in the unlikely event that a local authority specifies 
a non-minute UTC offset, if we go to that authority and say something 
like this:

"We can implement two possibilities:

"(A) Your timestamps will be unambiguous, but this will complicate some 
obscure calculations on old-fashioned platforms during the trailing part 
of a leap-second minute.

"(B) We can simplify those obscure calculations, but your timestamps 
will be ambiguous."

then the local authority will pick (A). I doubt it'll pick (B).

Of course I could be wrong. And it's also possible that a local 
authority would pick something that's neither (A) nor (B), though it's 
unclear what that would be (apparent solar time, perhaps?). From what we 
know now, though, I think we're better off implementing (A) than 
implementing (B).

If this unlikely event ever occurs and local authorities choose 
something other than (A), I hope we can adjust tzdb to what they 
actually do, much as we already adjust tzdb for other occasions where 
local authorities make unexpected changes. (If they choose apparent 
solar time I expect we'll be out of luck though....)

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