Return active times in HH:MM format

ee/app/graphql/types/incident_management/oncall_rotation_active_period_type.rb

@@ -9,13 +9,19 @@ module Types
 
       field :from, GraphQL::STRING_TYPE,
                 null: true,
-                description: 'The start of the rotation interval.',
-                method: :start_time
+                description: 'The start of the rotation interval.'
 
       field :to, GraphQL::STRING_TYPE,
                 null: true,
-                description: 'The end of the rotation interval.',
-                method: :end_time
+                description: 'The end of the rotation interval.'
+
+      def from
+        object.start_time&.strftime('%H:%M')
+      end
+
+      def to
+        object.end_time&.strftime('%H:%M')
+      end
     end
     # rubocop: enable Graphql/AuthorizeTypes
   end

ee/app/models/incident_management/oncall_rotation.rb

@@ -2,8 +2,6 @@
 
 module IncidentManagement
   class OncallRotation < ApplicationRecord
-    include Gitlab::Utils::StrongMemoize
-
     ActivePeriod = Struct.new(:start_time, :end_time) do
       def present?
         start_time && end_time