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


That's interesting problem indeed.
I'll look at it on particular example: my own timezone :)

We have GMT+2 at winter and GMT+3 at summer, transitions for 2018 were Mar 25 03:00 and Oct 28 04:00.

So....

For mid-period, everything is obvious, checking at UTC is enough:

```ruby
tz = TZInfo::Timezone.get('Europe/Kiev')

tz.utc_offset(Time.utc(2018, 1, 1))
# => 7200 
tz.utc_offset(Time.utc(2018, 6, 1))
# => 10800 
```

But how do we decide for the 2018-10-28 4:00 for example? I believe it could be done this way (with the only method `#utc_offset(at)` required):
```ruby
tz.utc_offset(Time.utc(2018, 10, 28, 2, 0)) # step 1: take "approximate" offset from UTC time with given value
# => 7200 
tz.utc_offset(Time.new(2018, 10, 28, 2, 0, 0, 7200)) # step 2: take real offset from time with approximate offset
# => 10800 
tz.utc_offset(Time.new(2018, 10, 28, 2, 0, 0, 10800)) # step 3: check it: yes, it is right
# => 10800 
```
So, the real answer is: Time at `2018-10-28 02:00 Europe/Kiev` has UTC offset `GMT+3`. 

Same for transition in another direction:
```ruby
tz.utc_offset(Time.new(2018, 3, 25, 3, 0, 0)) # "approximate" offset
# => 10800 
tz.utc_offset(Time.new(2018, 3, 25, 3, 0, 0, 10800)) # real offset?
# => 7200 
tz.utc_offset(Time.new(2018, 3, 25, 3, 0, 0, 7200)) # check it: no, it is transition point
# => 10800 
```
The real answer is: it is lost hour (we have 04:00 after 02:59 at transition point), `2018-10-25 03:00 Europe/Kiev` can't exist, should be an exception.

Yes, 3 calls to `utc_offset` are kinda indirect, but the current implementation is also "indirect" in a sense it requires timezone library to calculate `Time` object but doesn't use it. 

What is worse is: if modern (2.6-aware) timezone library will try to make _proper_ Time object (using `Time.new` with its timezone object), there could be infinite recursion (because `Time` itself and timezone library would call each other). That's because current requirements were designed with exactly one implementation in mind -- which is a third-party library with a legacy interface. 

In fact, it is funny paradox that exactly this "legacy" feature (library is able to work with non-offsetted time, considering it as just "tuple of time values"). Maybe more robust API to require would be something like:

```ruby
tz.utc_offset_by_tuple(2018, 3, 25, ...) # => consider it as a components of local time, return seconds offset
```

PS: In Ruby, currently, creating time on a border of transition is impossible
```ruby
Time.new(2018, 10, 28, 3, 00, 0, tz) 
# TZInfo::AmbiguousTime (2018-10-28 03:00:00 is an ambiguous local time.)
```

TZInfo itself solves it this way:
```ruby
tz.local_time(2018, 10, 28, 3, 0, 0, 0) 
# TZInfo::AmbiguousTime (2018-10-28 03:00:00 is an ambiguous local time.)
tz.local_time(2018, 10, 28, 3, 00, 0, 0, true) # last param is dst=true
# => 2018-10-28 03:00:00 +0300 
tz.local_time(2018, 10, 28, 3, 00, 0, 0, false) # dst = false
# => 2018-10-28 03:00:00 +0200 
```

----------------------------------------
Feature #15527: Redesign of timezone object requirements
https://bugs.ruby-lang.org/issues/15527#change-76633

* 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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