i18n-and-timezones

I18n + Time Zones

Two related areas Rails handles well by default but AI agents botch consistently. Strings get hardcoded. Time.now returns server-local instead of UTC. DST changes break appointment booking. This skill encodes the defaults that prevent each class of bug.

The opinion

Externalize every user-visible string via t(). Use the rails-i18n gem for built-in locales (date formats, currency, validation messages). Store all times as UTC in the database (Rails default). Use Time.current / Time.zone.now, never Time.now. Set Time.zone per-request based on user preference. Use the time_in_zone helper for display. Test DST transitions explicitly.

Core patterns

Pattern 1: Externalize strings

# config/locales/en.yml
en:
  greetings:
    welcome: "Welcome, %{name}!"
    farewell: "See you soon."
  posts:
    index:
      title: "All Posts"
      new: "New Post"
      filter:
        all: "All"
        drafts: "Drafts only"

<h1><%= t(".title") %></h1>           <!-- Lazy lookup: app/views/posts/index → t("posts.index.title") -->
<%= link_to t(".new"), new_post_path %>
<%= t("greetings.welcome", name: current_user.name) %>

Lazy lookup (t(".title")) — Rails infers the namespace from the view path. Way less repetitive than full t("posts.index.title") in every line.

Pattern 2: Locale fallbacks

# config/application.rb
config.i18n.available_locales = %i[en fr de ja]
config.i18n.default_locale = :en
config.i18n.fallbacks = [I18n.default_locale]   # or true for chain fallback
config.i18n.fallbacks.map = { fr: :en, de: :en, ja: :en }  # explicit per-locale

# config/initializers/i18n.rb — enable Fallbacks backend (required for `fallbacks.map`)
require "i18n/backend/fallbacks"
I18n::Backend::Simple.include I18n::Backend::Fallbacks

Missing French translation → falls back to English. Never falls back to a missing key (translation missing placeholder).

For production, use config.i18n.raise_on_missing_translations = true in test env to fail specs that reference missing keys.