JS offset

# assumes a User model with restful_authentication and a string time_zone column

class ApplicationController < ActionController::Base
  around_filter :set_timezone

  def set_timezone
    TzTime.zone = TimeZone[ current_user && current_user.time_zone || session[:utc_offset_hours] || 0 ]
    yield
    TzTime.reset!
  end
end

class ApplicationHelper
  # print UTC time (eg record created_at timestamps) as localtime for the current user
  def tz(time)
    if current_user && current_user.time_zone
      TzTime.zone.utc_to_local(time.utc)
    else
      time.to_time.utc + (session[:utc_offset_hours] || 0).hours
    end
  end
end

// global.js

$(document).ready(function() {

  // set the local timezone as per the browser's idea of local time
  var t = new Date();
  // expects ENV.timezoneOffset to be returned by server once set in session
  // otherwise this post happens on each request
  if(ENV.timezoneOffset != t.getTimezoneOffset()) 
    $.post('/time_zone', { format: 'json', _method: 'put', utc_offset: t.getTimezoneOffset() });

});
# timezone controller accepts an initial JS post giving the offset, also allows the user to specify their 'real' timezone via a drop-down

class TimeZoneController < ApplicationController
  skip_before_filter :load_announcements, :only => ['update']
  skip_before_filter :verify_authenticity_token, :only => ['update']

  def update
    respond_to do |format|
      format.json do
        # session[:javascript_utc_offset] = params[:utc_offset];
        session[:utc_offset_hours] = (params[:utc_offset].to_i / 60).abs
        render :nothing => true, :status => 204, :content_type => 'application/json'
      end
      format.any do
        verify_authenticity_token
        if params[:time_zone] && TimeZone[params[:time_zone]]
          current_user.time_zone = params[:time_zone]
          current_user.save!
          flash[:notice] = "Thank you. Your time zone is now set to #{params[:time_zone]}"
          redirect_to '/'
        else
          flash[:warning] = "Failed to set time zone. Please try again."
          edit
          render :action => 'edit'
        end
      end
    end
  end

  def edit
    @time_zones = TimeZone.all.sort_by(&:utc_offset)
  end
end

# time_zone/edit.html.haml

%h3.green-type Set your local Time Zone

%form{:action => '/time_zone', :method => 'POST'}
  %input{:type => 'hidden', :name => 'authenticity_token', :value => form_authenticity_token}
  %input{:type => 'hidden', :name => '_method', :value => 'PUT'}

  %p
    %label{:for => 'time_zone'} Choose your local time zone from the list:
  %p.input
    %select{:name => 'time_zone'}
      - @time_zones.each do |zone|
        %option{:value => zone.name, :selected => zone == TzTime.zone}= zone

  %p.input
    %input.green{:type => 'submit', :value => 'Remember my Time Zone'}