Issue #15527 has been updated by zverok (Victor Shepelev).


> Note that a timezone system requires two API.

Sorry for my arrogance, but can you please explain this a bit?..
From what I can understand from code, `local_to_utc` is used only from `Time.new`, but I am not quite sure why exactly.
From "logical" point of view, we need just to understand what exact UTC offset it should have (so it covered with `Zone#utc_offset`), the only uncertainty here is *what exact moment* we want UTC offset for -- this can fire in the foot at DST change moment, but I am not sure how a pair of APIs could help here.

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Feature #15527: Redesign of timezone object requirements
https://bugs.ruby-lang.org/issues/15527#change-76314

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
In #14850, there was timezone support introduced, there were pretty specific requirements for the Timezone object:

> A timezone argument must have `local_to_utc` and `utc_to_local` methods... The `local_to_utc` method should convert a `Time`-like object from the timezone to UTC, and `utc_to_local` is the opposite. ... The zone of the result is just ignored. 

I understand this requirements were modelled after existing TZInfo gem, but the problem with them are:
* they are too ad-hoc (in fact, return values of methods aren't used as a "Time object", but as a tuple of time components)
* they belong to outdated tzinfo API (ignoring of offsets is due to support of **Ruby 1.8**, which didn't allowed constructing `Time` object with arbitrary offset, see [discussion](https://github.com/tzinfo/tzinfo/issues/49)), recent [release](https://github.com/tzinfo/tzinfo/pull/52) introduces also `#to_local`, which returns `Time` with proper offset.

The latter is a bit of time paradox: Ruby **2.6** new feature is designed after the library which works this way to support Ruby **1.8** :)
The bad thing is, this approach somehow "codifies" outdated API (so in future, any alternative timezone library should support pretty arbitrary API).

I believe, that in order to do everything that `Time` needs, _timezone_ object should be able to answer exactly one question: "what offset from UTC is/was observed in this timezone at particular date". In fact, TZInfo **has** the [API](https://www.rubydoc.info/gems/tzinfo/TZInfo/Timezone#observed_utc_offset-instance_method) for this:

```ruby
tz = TZInfo::Timezone.get('America/New_York')
# => #<TZInfo::DataTimezone: America/New_York> 
tz.utc_offset(Time.now)
# => -18000 
```

If I understand correctly, this requirement ("A timezone argument must have `#utc_offset(at_time)`") will greatly simplify the implementation of `Time`, while also being compatible with `TZInfo` gem and much more explainable. With this requirement, alternative implementations could now be much simpler and focus only on "find the proper timezone/period/offset", omitting any (hard) details of deconstructing/constructing Time objects.



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