Turbo Streams + Rails Time Zone Helpers: Date Fragment Updates

Answer

When a user changes their preferred time zone, the simplest and most reliable way to update date fragments that are rendered by Turbo Streams is to let the server re‑render the fragment in the new zone and send that markup in the Turbo Stream. Turbo Streams do not perform any client‑side conversion, so the HTML you send must already contain the correctly formatted timestamp.

Why a Stimulus rewrite is unnecessary

Stimulus would have to parse the rendered string, apply a JavaScript‑side time‑zone conversion, and replace the innerHTML of the node. That adds complexity, introduces a second source of truth for the format, and can easily get out of sync with the server‑side formatting rules. The Rails l helper (or time_ago_in_words, etc.) already produces the desired string when Time.zone is set, so re‑rendering on the server is the idiomatic Rails pattern.

Recommended pattern

1. Set Time.zone per request. In ApplicationController add a before_action that pulls the user’s zone from session[:time_zone] or a user attribute and assigns it:

   before_action :set_time_zone
   
   def set_time_zone
     Time.zone = current_user.time_zone.presence || "UTC"
   end
   

2. Render the fragment in the controller action that triggers the Turbo Stream. The action should render the partial that contains the date helper:

   def update
     @record = Record.find(params[:id])
     respond_to do |format|
       format.turbo_stream
     end
   end
   

   The app/views/records/_record.html.erb partial can use:

   <%= l @record.updated_at, format: :short %>
   

   Because Time.zone is already set, l outputs the timestamp in the user’s zone.
3. Invalidate cache when the zone changes. If you cache the partial (e.g. cache @record, @record.updated_at, current_user.time_zone), include the zone in the cache key so that a zone change forces a new cache entry.

Cache‑busting with dynamic zones

When you use turbo_stream_from or turbo_stream.update, the server can set a Cache‑Control header that expires quickly (e.g. max-age=0, must-revalidate). More robustly, include the user’s time zone in the cache key or in the Turbo Stream’s data-turbo-cache-key attribute. Example:

<%= turbo_stream.update "record_#{@record.id}", partial: "records/record", locals: { record: @record }, cache_key: "record_#{@record.id}_#{current_user.time_zone}" %>

This ensures that a change to session[:time_zone] will cause the next Turbo Stream to fetch fresh markup.

Missing diagnostic detail

To fine‑tune the cache key, I need to know how the user’s time zone is stored (session, user model, or cookie). Could you confirm that?