diff --git a/src/order_cmd.cpp b/src/order_cmd.cpp
--- a/src/order_cmd.cpp
+++ b/src/order_cmd.cpp
@@ -1845,13 +1845,21 @@ void DeleteVehicleOrders(Vehicle *v, boo
 
 /**
  * Clamp the service interval to the correct min/max. The actual min/max values
- * depend on whether it's in percent or days.
- * @param interval proposed service interval
- * @return Clamped service interval
+ * depend on whether it's in days, minutes, or percent.
+ * @param interval The proposed service interval.
+ * @param ispercent Whether the interval is a percent.
+ * @return The service interval clamped to use the chosen units.
  */
 uint16_t GetServiceIntervalClamped(int interval, bool ispercent)
 {
-	return ispercent ? Clamp(interval, MIN_SERVINT_PERCENT, MAX_SERVINT_PERCENT) : Clamp(interval, MIN_SERVINT_DAYS, MAX_SERVINT_DAYS);
+	/* Service intervals are in percents. */
+	if (ispercent) return Clamp(interval, MIN_SERVINT_PERCENT, MAX_SERVINT_PERCENT);
+
+	/* Service intervals are in minutes. */
+	if (TimerGameEconomy::UsingWallclockUnits(_game_mode == GM_MENU)) return Clamp(interval, MIN_SERVINT_MINUTES, MAX_SERVINT_MINUTES);
+
+	/* Service intervals are in days. */
+	return Clamp(interval, MIN_SERVINT_DAYS, MAX_SERVINT_DAYS);
 }
 
 /**