Setting the Time Zone Dynamically

[adeeb1]

I ran into the "Cannot determine timezone from nil" error and wanted to post my solution. It involves setting the ENV['TZ'] variable, as you suggested, but I don't like to hardcode values like this, especially if your time zone changes from development to production environments. You can let Ruby get the time zone more explicitly.

ENV['TZ'] = Time.zone.name
scheduler = Rufus::Scheduler.new
scheduler.every '2s' do
  puts "#{Time.now} Hello #{ENV['TZ']}!"
end

[jmettraux]

Good morning,

$ ruby -v
ruby 2.2.5p319 (2016-04-26 revision 54774) [x86_64-darwin14]
$ ruby -e "Time.zone.name"
-e:1:in `<main>': undefined method `zone' for Time:Class (NoMethodError)

[adeeb1]

Thanks for the quick reply! It looks like Time.zone is specific to ActiveSupport: http://api.rubyonrails.org/classes/ActiveSupport/TimeZone.html. I think my approach would work for Rails, but without Rails you'd need to manually specify the time zone as you have described in your FAQ.

[jmettraux]

What kind of zone string does your Time.zone.name returns?

If it's something like "CST" it is very ambiguous and leads to problems like #228

Beware of not shooting yourself in the foot.

[adeeb1]

Good point. Mine returns "UTC" and seems to be working fine. ActiveSupport::TimeZone is a wrapper over TZInfo. What may be even safer is to use the tzinfo identifier instead:

ENV['TZ'] = Time.zone.tzinfo.identifier # "Etc/UTC"

[jmettraux]

Could you please try something in your system for me?

Please set your system zone to Shangai then tell me what Time.zone.tzinfo.identifier returns.

Thanks in advance.

[adeeb1]

It doesn't look like it uses your local time; mine returns "UTC." The docs say you have to set the time zone in Rails' application.rb; otherwise it will default to UTC. Shanghai isn't listed as a time zone:

> rake time:zones:all
...

* UTC +08:00 *
Beijing
Chongqing
Hong Kong
Irkutsk
Kuala Lumpur
Perth
Singapore
Taipei
Ulaanbaatar
...

I set the time zone to "Beijing," and it returned the Shanghai identifier, which makes sense since it says "Beijing Time" next to it on Wikipedia.

> rails c
> Time.zone = "Beijing"
=> "Beijing"
> Time.zone.tzinfo.identifier
=> "Asia/Shanghai"

[jmettraux]

OK, how about changing the operating system zone?

[adeeb1]

The same as above. I changed the time zone of my OS, and it still returned "UTC" because Rails defaults to UTC if you don't explicitly specify a time zone in your Rails app.

[jmettraux]

OK. Thank you very much!

[jmettraux]

(removed the [FAQ] prefix as it might confuse people)