diff --git a/src/vehicle.cpp b/src/vehicle.cpp
--- a/src/vehicle.cpp
+++ b/src/vehicle.cpp
@@ -1369,7 +1369,7 @@ bool Vehicle::HandleBreakdown()
  */
 void AgeVehicle(Vehicle *v)
 {
-	if (v->age < MAX_DATE) {
+	if (v->age < CalendarTime::MAX_DATE) {
 		v->age++;
 		if (v->IsPrimaryVehicle() && v->age == VEHICLE_PROFIT_MIN_AGE + 1) GroupStatistics::VehicleReachedMinAge(v);
 	}
@@ -1378,7 +1378,7 @@ void AgeVehicle(Vehicle *v)
 
 	auto age = v->age - v->max_age;
 	for (int32_t i = 0; i <= 4; i++) {
-		if (age == DateAtStartOfYear(i)) {
+		if (age == TimerGameCalendar::DateAtStartOfYear(i)) {
 			v->reliability_spd_dec <<= 1;
 			break;
 		}
@@ -1396,11 +1396,11 @@ void AgeVehicle(Vehicle *v)
 	if (EngineHasReplacementForCompany(c, v->engine_type, v->group_id)) return;
 
 	StringID str;
-	if (age == DateAtStartOfYear(-1)) {
+	if (age == TimerGameCalendar::DateAtStartOfYear(-1)) {
 		str = STR_NEWS_VEHICLE_IS_GETTING_OLD;
-	} else if (age == DateAtStartOfYear(0)) {
+	} else if (age == TimerGameCalendar::DateAtStartOfYear(0)) {
 		str = STR_NEWS_VEHICLE_IS_GETTING_VERY_OLD;
-	} else if (age > DateAtStartOfYear(0) && (static_cast<int32_t>(age) % DAYS_IN_LEAP_YEAR) == 0) {
+	} else if (age > TimerGameCalendar::DateAtStartOfYear(0) && (static_cast<int32_t>(age) % CalendarTime::DAYS_IN_LEAP_YEAR) == 0) {
 		str = STR_NEWS_VEHICLE_IS_GETTING_VERY_OLD_AND;
 	} else {
 		return;