One of the things that I found is that the original TZInfo GEM has more zones, so the TZInfo::Timezone.get call retrieves more zones (e.g. Europe/Oslo). I kept this GEM before I converted my data and after that I removed it (of course I want as little external stuff as needed).

But I also added something to select at least the same time zone (with a different city) in case the old time zone can't be found in TimeZone::MAPPING.

My migration looks like this (maybe it helps others), based on the example above. By the way, I kept the old time zone in a backup field, to make a backwards conversion possible (for the time being, that is).

def self.up

# convert all user time zones

change_column :users, :timezone, :string, :limit => 40, :null => false, :default => 'London'

add_column :users, ld_timezone, :string, :limit => 40, :null => false, :default => 'Europe/London'

execute 'UPDATE users SET old_timezone = timezone'

User.all.each do |user|

tz = TZInfo::Timezone.get(user.timezone) rescue TimeZone[user.timezone] || TimeZone['UTC']

if tz.is_a?(TZInfo::Timezone)

linked_timezone = tz.instance_variable_get('@linked_timezone')

name = linked_timezone ? linked_timezone.name : tz.name

time_zone = TimeZone::MAPPING.index(name)

if time_zone.nil?

compatible_zones = TimeZone.all.select { |t| t.utc_offset == tz.current_period.offset.utc_total_offset }

time_zone = compatible_zones.empty? ? 'UTC' : compatible_zones.first.name

end

else

time_zone = tz.name

end

user.update_attribute :timezone, time_zone

end

def self.down

execute 'UPDATE users SET timezone = old_timezone'

remove_column :users, ld_timezone

change_column :users, :timezone, :string, :limit => 40, :null => false, :default => 'Europe/London'

end

(My field in the users table is called timezone).

You could change it's default to UTC. It doesn't have to do an update for every user record.

"You can find it here."http://www.jrmiii.com/2008/6/3/quick-migration-to-set-user-time-zones-for-rails-2-1

Cheers