[tz] On merging timezones - a radical proposal.

[Guy Harris]

On May 22, 2013, at 1:14 PM, random832 at fastmail.us wrote:

> On Wed, May 22, 2013, at 15:24, Guy Harris wrote:
> 
>> Presumably if users can "pick advanced", the *system vendors* have to
>> ship the full set of time zone files (otherwise, "advanced" isn't
>> available).
> 
> I am not proposing reducing the timezone files that are shipped, merely
> adding a way to have fewer timezones to choose from in the UI.

The only UI we ship is tzselect; nothing we do prevents vendors from providing a UI that works the way you suggest on top of our current database.

>> We don't handle that because UN*X APIs don't offer a mechanism for saying
>> "please use the time zone I was in at a given time to convert that time";
>> if they did, they could use our database for that.
> 
> Why is that not what localtime() means?

Because nobody's ever bothered to implement that behavior.

> And if it _doesn't_ mean that,
> why doesn't localtime simply _always_ use Eastern for people in
> Vincennes, despite the fact that "the time zone they were in at a given
> time" was Central for part of 2006-2007?

Why should it?  "localtime() doesn't know where you happened to be at some time in the past" does not imply "localtime() shouldn't care whether where you are *now* happens to have changed its {time zone offset, DST rules, time zone name} at some time in the past".