Generic Partial Location

[sffc]

Generic Partial Location is supposed to be used when the offsets for a zone are ambiguous.

There isn't a clear algorithm in the spec on how to determine this, but I assume one would be roughly:

1. Compute the offset from the TZDB for the golden zone of the metazone in the current region
2. If the given offset differs from the computed offset, use Generic Partial Location
3. Otherwise, if the offsets match, use Generic Non-Location

For example: It is July 1 in America/Phoenix. The metazone is America_Mountain. The golden zone is America/Denver. The offset for America/Denver on July 1 is -6. However, America/Phoenix has an offset of -7. Therefore, use Generic Partial Location.

However, we can't do this type of calculation at formatting time, since it definitely needs the TZDB.

Could we do it via ZoneVariant?

• ZoneVariant::Standard
• ZoneVariant::StandardStrange
• ZoneVariant::Daylight
• ZoneVariant::DaylightStrange

and if "StandardStrange" or "DaylightStrange" are used, then we print Generic Partial Location instead of Generic Non-Location when a Generic format is requested.

I think we could probably make the zone variant calculator figure this out, but I haven't thought through all the edge cases yet.

@robertbastian

[sffc]

I'm not sure if the zone variant calculator is enough. It is only supposed to know about when a time zone switched between metazone or zone variants with a broad stroke, not time zone transitions. I don't see how it could answer the question, "does this time in America/Phoenix differ from the time in America/Denver?" Unless there is a way? I hope I'm missing something.

[robertbastian]

The only case where this can happen is if a zone has a different DST offset than the golden zone: https://unicode.org/reports/tr35/tr35-dates.html#goals:~:text=Except%20for%20daylight%20savings%2C%20at%20any%20given%20time%2C%20all%20zones%20in%20a%20metazone%20have%20the%20same%20offset%20at%20that%20time.

We can add a non_standard_dst flag to MetazonePeriodsV1, and then if zone_variant == ZoneVariant::daylight() && metazone.non_standard_dst -> use generic partial location

[sffc]

Oh, hmm, yeah that might work, although it would mean that you get "Mountain Time (Denver)", so basically one bad apple spoils the format for all time zones in that metazone.

[robertbastian]

Ah actually America/Phoenix will never have ZoneVariant::daylight(). We would need to add the flag to ZoneOffsetPeriodsV1, so that we can determine whether the zone variant is standard regular.

[robertbastian]

Oh, hmm, yeah that might work, although it would mean that you get "Mountain Time (Denver)", so basically one bad apple spoils the format for all time zones in that metazone.

America/Denver is the golden zone, so it itself will not be considered non-standard.

[sffc]

Maybe we just mark America/Phoenix as "strange" (within a certain period of time) and print "Mountain Time (Phoenix)" all year round?

[sffc]

We probably shouldn't be the ones deciding this. Maybe bring it to CLDR Design WG.

[robertbastian]

Maybe we just mark America/Phoenix as "strange" (within a certain period of time) and print "Mountain Time (Phoenix)" all year round?

It would have to be all year round, because without the TZDB we cannot tell whether the golden zone is currently observing DST. We can only tell if an input zone is observing DST because we get an input offset, for the golden zone there is no way for us to get this information.

We probably shouldn't be the ones deciding this. Maybe bring it to CLDR Design WG.

CLDR wants us to use the TZDB for this.

[sffc]

We probably shouldn't be the ones deciding this. Maybe bring it to CLDR Design WG.

CLDR wants us to use the TZDB for this.

CLDR only says: "when the generic non-location format is not specific enough" which very non-specific.

[sffc]

If TZDB is really the only way to do this, it's not completely out of the picture to make our own annotation in IXDTF. It is allowed in the spec for annotations to be extensible. Something like

2024-10-02T15:05:51-0700[America/Phoenix][u-td=yes]

where u-td=yes means roughly "Unicode Time zone Disambiguation: Yes"

[robertbastian]

CLDR wants us to use the TZDB for this.

CLDR only says: "when the generic non-location format is not specific enough" which very non-specific.

* I assume CLDR will tell us to use TZDB for this.

If TZDB is really the only way to do this, it's not completely out of the picture to make our own annotation in IXDTF.

Not a fan of this. This would require it to be an additional field of whatever we settle on in #5533.

[sffc]

A question Mark asked last time we briefly discussed this, IIRC, was "check what ICU4C does". I haven't checked yet but we should do that.

[robertbastian]

ICU requires a full TZDB, so they can do the clever thing. We don't want to ship a full TZDB.

[sffc]

ICU requires a full TZDB, so they can do the clever thing. We don't want to ship a full TZDB.

Yes, but what are they actually doing?

[sffc]

Not a fan of this. This would require it to be an additional field of whatever we settle on in #5533.

If the condition isn't derivable from offset plus time zone identity, it's going to need to go somewhere in the input schema.

[robertbastian]

Only if we want to do the fancy thing of only giving Phoenix special treatment while Denver is observing DST. If we special-case it year-round that can be derived from the time zone identity.

[sffc]

Well, even in that case, don't we still need it in the input schema? Either as a unique zone variant, metazone, or additional field?

[sffc]

Well, even in that case, don't we still need it in the input schema? Either as a unique zone variant, metazone, or additional field?

[robertbastian]

We can mark America/Phoenix as weird in the metazone lookup. We have spare bits in the ASCII encoding. For example we could encode Ammt instead of ammt for "belongs to metazone, but needs partial location format".

[sffc]

I'm assuming we land on the approach in #5533 where the metazone and zone variant remain calculated by icu_timezone, meaning this would still need to be in the interface between FormattableTimeZone and datetime::Formatter

[robertbastian]

This is actually way more complex: https://unicode.org/reports/tr35/tr35-dates.html#Contents:~:text=Otherwise%20do%20the,Pacific%20Time%20(Whitehorse)%22

[sffc]

Hmm, I must have overlooked that algorithm in the spec. Good news though is that it is based on time zone identity rather than TZDB.