Setting I18n localizations to format times based on user's timezone

application_controller.rb

class ApplicationController < ActionController::Base
  around_filter :set_i18n_locale_from_current_user

  def set_i18n_locale_from_current_user
    org_timezone = I18n.current_timezone
    if current_user
      I18n.current_timezone = current_user.timezone
    end
    yield
    ensure
      I18n.current_timezone = org_timezone
  end
end

date.rb

# From
# https://gist.github.com/alexisbernard/179712
# with updates including 
# _date_format(str, format)
# to allow /, - or . as separators 
# AND
# match ruby params for _parse && parse
# https://gist.github.com/itchy/6383770

class Date
  class << self
    def _date_format(str, format)
      format ||= :default
      date_format = I18n.translate("date.formats.#{format}")
      date_format[/([\.\/-])/]
      format_seperator = $1
      str[/([\.\/-])/]
      str_seperator = $1
      date_format.gsub!(format_seperator, str_seperator) if format_seperator && str_seperator
      date_format
    end

    def _parse_with_i18n(str, comp=true, format = :default)
      date_format = _date_format(str, format)

      date = Date._strptime(str, date_format) || _parse_without_i18n(str)
      date = _parse_without_i18n(str) if date[:leftover]
      date[:year] += increment_year(date[:year].to_i) if date[:year]
      date
    end
 
    alias_method_chain :_parse, :i18n
 
    def parse_with_i18n(str, comp=true, start=ITALY, format = :default)
      date_format = _date_format(str, format)

      date = Date.strptime(str, date_format)
      Date.new(date.year + increment_year(date.year), date.month, date.day)
    rescue ArgumentError
      parse_without_i18n(str)
    end
 
    alias_method_chain :parse, :i18n
 
    def increment_year(year)
      if year < 100
        year < 30 ? 2000 : 1900
      else
        0
      end
    end
  end
end

i18n.rb

# in initializers
module I18n
  def self.current_timezone=(tz='UTC')
    Thread.current[:current_time_zone] = ActiveSupport::TimeZone.find_tzinfo(tz)
  end

  def self.current_timezone
    Thread.current[:current_time_zone] || ActiveSupport::TimeZone.find_tzinfo('UTC')
  end

  module Backend::Base
    def localize_with_timezone(locale, object, format = :default, options = {})
      raise ArgumentError, "Object must be a Date, DateTime or Time object. #{object.inspect} given." unless object.respond_to?(:strftime)
      object = object.in_time_zone(I18n.current_timezone) if object.respond_to?(:in_time_zone)
      localize_without_timezone(locale, object, format, options = {})
    end

    alias_method_chain :localize, :timezone
  end
end

I wonder if it is thread safe. Variable @current_time_zone seems to be a global one because of self. However, I have not looked for it, but if the I18n.locale is implemented the same way we should do so too.

Also function should be named set_i18n_timezone_from_current_user and not replace the set_I18n_local_from_current_user (typo?).

What you do at module Backend::Base looks interesting, but that all I can say here.

Thanks Jens -- I updated the gist to have I18n#current_timezone be thread safe

Update date.rb to match Ruby args