Add clarifying comments on "with" or "in" time_zone

Frontend->Backend: We don't have timezone so we have to attach timezone from browser
Backend->Frontend: We have timezone UTC and need to convert it to the
browser timezone before displaying

app/models/banner.rb

@@ -12,6 +12,8 @@ def expired?
     expires_at && Time.current > expires_at
   end
 
+  # `expires_at` is stored in the database as UTC, but timezone information will be stripped before displaying on frontend
+  # so this method converts the time to the user's timezone before displaying it
   def expires_at_in_time_zone(timezone)
     expires_at&.in_time_zone(timezone)
   end

app/values/banner_parameters.rb

@@ -4,15 +4,18 @@ def initialize(params, user, timezone)
     new_params = params.require(:banner).permit(:active, :content, :name, :expires_at).merge(user: user)
 
     if params.dig(:banner, :expires_at)
-      new_params[:expires_at] = convert_expires_at_in_user_time_zone(params, timezone)
+      new_params[:expires_at] = convert_expires_at_with_user_time_zone(params, timezone)
     end
 
     super(new_params)
   end
 
   private
 
-  def convert_expires_at_in_user_time_zone(params, timezone)
+  # `expires_at` comes from the frontend without any timezone information, so we use `in_time_zone` to attach
+  # timezone information to it before saving to the database. If we don't do this, the time will be stored at UTC
+  # by default.
+  def convert_expires_at_with_user_time_zone(params, timezone)
     params[:banner][:expires_at].in_time_zone(timezone)
   end