diff --git a/src/engine_func.h b/src/engine_func.h
--- a/src/engine_func.h
+++ b/src/engine_func.h
@@ -27,7 +27,7 @@ bool IsEngineBuildable(EngineID engine, 
 bool IsEngineRefittable(EngineID engine);
 void SetYearEngineAgingStops();
 void CalcEngineReliability(Engine *e, bool new_month);
-void StartupOneEngine(Engine *e, TimerGameCalendar::Date aging_date, uint32_t seed);
+void StartupOneEngine(Engine *e, const TimerGameCalendar::YearMonthDay &aging_ymd, uint32_t seed);
 
 uint GetTotalCapacityOfArticulatedParts(EngineID engine);