[ruby-core:93491] [Ruby master Bug#15977] Unobvious result value from #next_rotate_time and #previous_period_end methods of Logger::Period module

Issue #15977 has been reported by davydov_anton (Anton Davydov).

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Bug #15977: Unobvious result value from #next_rotate_time and #previous_period_end methods of Logger::Period module
https://bugs.ruby-lang.org/issues/15977

* Author: davydov_anton (Anton Davydov)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.5 - master
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
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I found that #next_rotate_time and #previous_period_end methods can return different time objects in some case. 

When we call these methods with valid `shift_age` option methods returns a new Time object with the local machine's timezone. If you put invalid `shift_age` argument it will return existed time object with any timezone. Script for better understanding and reproducing:

``` ruby
require 'time'
require 'logger'

time = Time.parse("2019-07-18 13:52:02 -0300")

result = Logger::Period.next_rotate_time(time, 'daily')
invalid_arg_result = Logger::Period.next_rotate_time(time, 'invalid')

p result.zone == invalid_arg_result.zone
p result.zone
p invalid_arg_result.zone
```

In this case, `result` object will have my local timezone and `invalid_arg_result` will have `-03` timezone.


I think that the problem in these lines:

https://github.com/ruby/ruby/blob/master/lib/logger.rb#L635
and
https://github.com/ruby/ruby/blob/master/lib/logger.rb#L654


The question is: is it good and if not what the way we can use to fix it?




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