[ruby-core:91386] [Ruby trunk Feature#15527] Redesign of timezone object requirements

[nobu@ruby-lang.org]

Issue #15527 has been updated by nobu (Nobuyoshi Nakada).


zverok (Victor Shepelev) wrote:
> 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. 

It is trivial, and is possible to change.
3 calls to `utc_offset` seems slower than 1 call to `local_to_utc`.

> 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).

You mean the case a timezone library calls `Time.new` in `utc_to_local` method?
`Time.new` with a timezone object would call `local_to_utc`, with a UTC time-like object, and the result should be UTC.

> That's because current requirements were designed with exactly one implementation in mind -- which is a third-party library with a legacy interface.

It was designed with another implementation, timezone gem, too.

> 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
> ```

Non-offsetted Time-like object is used now.

> 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 
> ```

Yes I know, but another implementation in my mind, timezone, doesn't support `dst` argument.

----------------------------------------
Feature #15527: Redesign of timezone object requirements
https://bugs.ruby-lang.org/issues/15527#change-76642

* 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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