diff --git a/library/src/calculate_price.cpp b/library/src/calculate_price.cpp
index b021c62..0a70495 100644
--- a/library/src/calculate_price.cpp
+++ b/library/src/calculate_price.cpp
@@ -342,7 +342,7 @@ CalcState CALCULATE_LIBRARY_API compute_price_for_parking_ticket(
 
 CalcState CALCULATE_LIBRARY_API compute_price_for_parking_ticket(
         parking_tariff_t *tariff,
-        QDateTime const &start_parking_time,
+        QDateTime &start_parking_time,
         int netto_parking_time,
         QDateTime &end_parking_time,
         struct price_t *price)