diff --git a/app/controllers/api/v1/markers_controller.rb b/app/controllers/api/v1/markers_controller.rb
index 8eaf7767df8..cfb708ff308 100644
--- a/app/controllers/api/v1/markers_controller.rb
+++ b/app/controllers/api/v1/markers_controller.rb
@@ -32,13 +32,7 @@ class Api::V1::MarkersController < Api::BaseController
   private
 
   def serialize_map(map)
-    serialized = {}
-
-    map.each_pair do |key, value|
-      serialized[key] = ActiveModelSerializers::SerializableResource.new(value, serializer: REST::MarkerSerializer).as_json
-    end
-
-    Oj.dump(serialized)
+    map.transform_values { |value| ActiveModelSerializers::SerializableResource.new(value, serializer: REST::MarkerSerializer) }
   end
 
   def resource_params
diff --git a/spec/requests/api/v1/markers_spec.rb b/spec/requests/api/v1/markers_spec.rb
index 0e6ecc56855..17f19872f73 100644
--- a/spec/requests/api/v1/markers_spec.rb
+++ b/spec/requests/api/v1/markers_spec.rb
@@ -7,8 +7,10 @@ RSpec.describe 'API Markers' do
 
   describe 'GET /api/v1/markers' do
     before do
-      Fabricate(:marker, timeline: 'home', last_read_id: 123, user: user)
-      Fabricate(:marker, timeline: 'notifications', last_read_id: 456, user: user)
+      travel_to DateTime.parse('2026-03-15T12:34:56.789Z'), with_usec: true do
+        Fabricate(:marker, timeline: 'home', last_read_id: 123, user: user)
+        Fabricate(:marker, timeline: 'notifications', last_read_id: 456, user: user)
+      end
 
       get '/api/v1/markers', headers: headers, params: { timeline: %w(home notifications) }
     end
@@ -23,6 +25,11 @@ RSpec.describe 'API Markers' do
           notifications: include(last_read_id: '456')
         )
     end
+
+    it 'uses a specific style of IS08601 timestamps' do
+      expect(response.parsed_body)
+        .to include(home: include(updated_at: eq('2026-03-15T12:34:56.789Z')))
+    end
   end
 
   describe 'POST /api/v1/markers' do