Issue #14879 has been updated by ioquatix (Samuel Williams).


By the way, perhaps it's not clear, but `utc_offset` is not a time zone, nor is `Time.zone`, because just stating `MST` is not enough to disambiguate. You need to specify `Canada/Saskatchewan` (MST only) or `Canada/Mountain` (MST/MDT). Practically speaking, the best computation we have for `utc_offset` is the time zone database (i.e. government rules), a specific date+time in UTC, and a specific zone name from said database. That can then give us the `utc_offset`. Since several time zones might map to the same UTC offset, it's not easy to go in reverse without additional help.

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Bug #14879: Time#+ and Time#- do not preserve receiver's utc_offset if ENV['TZ'] is modified after receiver is created
https://bugs.ruby-lang.org/issues/14879#change-72727

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
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I have been having some problems with `Time`. When I add or subtract seconds, sometimes the utc_offset is changed unexpectedly.

```
$ ruby -e 'require "time"; puts Time.parse("5pm NZT")'            
2018-06-29 17:00:00 +1200

$ ruby -e 'require "time"; puts Time.parse("5pm NZT") + 1'
2018-06-29 17:00:01 +1200

$ TZ=UTC ruby -e 'require "time"; puts Time.parse("5pm NZT") + 1'
2018-06-29 17:00:01 +0000
```

This seems like strange behaviour. The `utc_offset` shouldn't change IMHO.



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