Fix flaky spec by setting a fixed date

This addresses flakiness to do with daylight savings.

ee/spec/services/incident_management/oncall_rotations/create_service_spec.rb

@@ -8,7 +8,9 @@
   let_it_be(:user_with_permissions) { create(:user) }
   let_it_be(:user_without_permissions) { create(:user) }
   let_it_be(:current_user) { user_with_permissions }
-  let_it_be(:starts_at) { Time.current.change(usec: 0) }
+
+  let(:execution_time) { DateTime.new(2021, 3, 1, 4, 5, 6) }
+  let(:starts_at) { DateTime.new(2021, 3, 1) }
 
   let(:participants) do
     [
@@ -40,7 +42,9 @@
       end
     end
 
-    subject(:execute) { service.execute }
+    subject(:execute) do
+      travel_to(execution_time) { service.execute }
+    end
 
     context 'when the current_user is anonymous' do
       let(:current_user) { nil }