Ensure datetime/date are serialized to correct format (#33086)

spec/serializers/rest/notification_group_serializer_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe REST::NotificationGroupSerializer do
+  subject do
+    serialized_record_json(
+      notification_group,
+      described_class
+    )
+  end
+
+  let(:notification_group) { NotificationGroup.new pagination_data: { latest_notification_at: 3.days.ago }, notification: Fabricate(:notification), sample_accounts: [] }
+
+  context 'when latest_page_notification_at is populated' do
+    it 'parses as RFC 3339 datetime' do
+      expect { DateTime.rfc3339(subject['latest_page_notification_at']) }.to_not raise_error
+    end
+  end
+end