diff --git a/src/engine.cpp b/src/engine.cpp
index 65d9d77379..0cc6e9cf52 100644
--- a/src/engine.cpp
+++ b/src/engine.cpp
@@ -611,8 +611,8 @@ void CalcEngineReliability(Engine *e, bool new_month)
 		re = Engine::Get(re->info.variant_id);
 	}
 
-	uint age = re->age;
-	if (new_month && re->index > e->index && age != MAX_DAY) age++; /* parent variant's age has not yet updated. */
+	uint32 age = re->age;
+	if (new_month && re->index > e->index && age != INT32_MAX) age++; /* parent variant's age has not yet updated. */
 
 	/* Check for early retirement */
 	if (e->company_avail != 0 && !_settings_game.vehicle.never_expire_vehicles && e->info.base_life != 0xFF) {
@@ -1094,7 +1094,7 @@ void EnginesMonthlyLoop()
 		bool refresh = false;
 		for (Engine *e : Engine::Iterate()) {
 			/* Age the vehicle */
-			if ((e->flags & ENGINE_AVAILABLE) && e->age != MAX_DAY) {
+			if ((e->flags & ENGINE_AVAILABLE) && e->age != INT32_MAX) {
 				e->age++;
 				CalcEngineReliability(e, true);
 				refresh = true;
diff --git a/src/engine_base.h b/src/engine_base.h
index d1bb8a3e0a..a05e8ee98f 100644
--- a/src/engine_base.h
+++ b/src/engine_base.h
@@ -37,7 +37,7 @@ extern EnginePool _engine_pool;
 struct Engine : EnginePool::PoolItem<&_engine_pool> {
 	std::string name;           ///< Custom name of engine.
 	TimerGameCalendar::Date intro_date; ///< Date of introduction of the engine.
-	TimerGameCalendar::Date age;        ///< Age of the engine, in days.
+	int32 age;                  ///< Age of the engine in months.
 	uint16 reliability;         ///< Current reliability of the engine.
 	uint16 reliability_spd_dec; ///< Speed of reliability decay between services (per day).
 	uint16 reliability_start;   ///< Initial reliability of the engine.