Issue #15988 has been reported by zverok (Victor Shepelev).

----------------------------------------
Bug #15988: Time#dst? vs "real" timezones
https://bugs.ruby-lang.org/issues/15988

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.6.3
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
My current timezone is Europe/Kiev.

With default zone (how it was previously):
```ruby
summer = Time.new(2019, 6, 1, 14, 30, 12)  # => 2019-06-01 14:30:12 +0300 
summer.zone                                # => "EEST" 
summer.dst?                                # => true 

winter = Time.new(2019, 12, 1, 14, 30, 12) # => 2019-12-01 14:30:12 +0200 
winter.zone                                # => "EET" 
winter.dst?                                # => false 
```
(Note it is correctly `true` in summer and `false` in winter.)

With "real" timezone object
```ruby
require 'tzinfo'
z = TZInfo::Timezone.get('Europe/Kiev')
# => #<TZInfo::DataTimezone: Europe/Kiev> 

summer = Time.new(2019, 6, 1, 14, 30, 12, z)  # => 2019-06-01 14:30:12 +0300 
summer.zone                                   # => #<TZInfo::DataTimezone: Europe/Kiev> 
summer.dst?                                   # => true 

winter = Time.new(2019, 12, 1, 14, 30, 12, z) # => 2019-12-01 14:30:12 +0200 -- offset is correct
winter.zone                                   # => #<TZInfo::DataTimezone: Europe/Kiev>
winter.dst?                                   # => true -- this is wrong!
```
Note that offsets are calculated correctly (+3 in summer, +2 in winter), but `dst?` is always `true`.

TZInfo itself calculates DST properly:
```ruby
z.dst?(summer) # => true 
z.dst?(winter) # => false 
```

Seems like a serious bug to me (or at least very hard to explain behavior).

PS: With arithmetics, everything works correctly:

```ruby
six_months = 6 * 30 * 24 * 60 * 60
summer + six_months                           # => 2019-11-28 13:30:12 +0200
(summer + six_months).dst?                    # => false -- that is correct
```



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