Blog of Ryan Bigg - How Rails Works #1: A Timezone Overview

31 Dec 2008

Today fowlduck and I were talking in the #rubyonrails channel and we both wondered about timezones and why they were so (apparently) screwy. It turns out all I (and he possibly also) forgot to do was to put config.time_zone in the config/environment.rb. So what does this mysterious method do? Well:

In Rails 2.2...

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  <li>1. <b>The <span class='term'>Configuration</span> Class</b><br />
This class begins all the way down on <a href='http://github.com/rails/rails/blob/2-2-stable/railties/lib/initializer.rb#L578'>line #578 of <i>railties/lib/initializer.rb</i></a> in the Rails source. This just simply defines a class in the <i>Rails</i> module called <i>Configuration</i>. What we can get really excited about is on <a href='http://github.com/rails/rails/blob/2-2-stable/railties/lib/initializer.rb#L748'>line #748</a> it defines an <span class='term'><a href='http://www.ruby-doc.org/core/classes/Module.html#M001704'>attr_accessor</a></span> for <span class='term'>:time_zone</span>. This, as you probably already know defines two methods a setter (<span class='term'>time_zone=</span>) and a getter (<span class='term'>time_zone</span>) in which we can store values.
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2. Your config/environment.rb file
By default this time_zone method will be set to nil. It's up to you to set it in your config/environment.rb file which you do by doing something along these lines:

Rails::Initializer.run do |config|
  config.time_zone = "Adelaide"
end

This will set the time_zone value to be the Adelaide Time zone, something like: #<ActiveSupport::TimeZone:0x30f4f8 @tzinfo=nil, @name="Adelaide", @utc_offset=34200>. You don't have to set it to Adelaide, just try your nearest major city and it Should Just Work ™. If you don't set this in your config/environment.rb date and time values returned from the database will not be set to whatever time zone you specify.

3. Back to you, Jeff. When your application loads it processes the config/environment.rb file and runs the initialize_time_zone method which is defined further down. This does all kinds of magic! Look at all the pretty sparkles! It firstly checks if you've set a time_zone in your config/environment.rb file and then if you have it sets the default time zone to be what you've specified. Additionally to this, it sets time_zone_aware_attributes to true so that when you do stuff like Topic.last.created_at it'll actually return the time zoned version of that time. It does this by calling into play define_read_method_for_time_zone_conversion(attr_name) (click for juicy details) which either just returns the time or calls in_time_zone on the time returned which converts it into the time zone stored in Time.zone (which is actually Thread.current[:time_zone] if there is one stored there or otherwise the zone_default which was originally set when we called config.time_zone! What a mouthful! By default, ActiveRecord will store timestamps as UTC as shown by ActiveRecord::Base.default_timezone = :utc. If you don't specify a time zone in your config/environment.rb this value defaults to :local, so all times will be stored as the local time in your database.

4. And then... So, assuming you did as the above when you go into your script/console and type: Topic.last.created_at you'll get back the time when the topic was created relative to Adelaide. To change this, just redefine Time.zone by doing Time.zone= "Paris" and then doing Topic.last.created_at will give you time when the topic was created relative to Paris.

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Changelog

Updated on April 23rd, 2009

Fixed line number linkings, linking directly to 2-2-stable branch which, ideally, should now never change.

The Life of a Radar

In Rails 2.2...

Changelog