ActionMailer Email Preview - show date header when present or fallback to Time.current.rfc2822

Resolves rails#49788

Author: sampatbadhe

railties/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   In Action Mailer previews, show date from message `Date` header if present.
+
+    *Sampat Badhe*
+
 *   Exit with non-zero status when the migration generator fails.
 
     *Katsuhiko YOSHIDA*

railties/lib/rails/templates/rails/mailers/email.html.erb

@@ -93,7 +93,7 @@
     <% end %>
 
     <dt>Date:</dt>
-    <dd id="date"><%= Time.current.rfc2822 %></dd>
+    <dd id="date"><%= @email.header['date'] || Time.current.rfc2822 %></dd>
 
     <dt>Subject:</dt>
     <dd><strong id="subject"><%= @email.subject %></strong></dd>

railties/test/application/mailer_previews_test.rb

@@ -989,6 +989,66 @@ def foo
       assert_match "<dd id=\"bcc\">[email protected]</dd>", last_response.body
     end
 
+    test "mailer preview date tag renders date from message header" do
+      mailer "notifier", <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "[email protected]"
+
+          def foo
+            mail to: "[email protected]", date: Time.utc(2023, 10, 20, 10, 20, 30)
+          end
+        end
+      RUBY
+
+      text_template "notifier/foo", <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview "notifier", <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app("development")
+
+      get "/rails/mailers/notifier/foo"
+      assert_match "<dd id=\"date\">Fri, 20 Oct 2023 10:20:30 +0000</dd>", last_response.body
+    end
+
+    test "mailer preview date tag falls back to current time when date header is not present" do
+      mailer "notifier", <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "[email protected]"
+
+          def foo
+            mail to: "[email protected]"
+          end
+        end
+      RUBY
+
+      text_template "notifier/foo", <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview "notifier", <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app("development")
+
+      travel_to(Time.utc(2023, 10, 20, 10, 20, 30)) do
+        get "/rails/mailers/notifier/foo"
+      end
+      assert_match "<dd id=\"date\">Fri, 20 Oct 2023 10:20:30 +0000</dd>", last_response.body
+    end
+
     test "mailer preview has access to rendering context" do
       mailer "notifier", <<-RUBY
         class Notifier < ActionMailer::Base