diff --git a/library/src/calculate_price.cpp b/library/src/calculate_price.cpp
index 0125777..58363a6 100644
--- a/library/src/calculate_price.cpp
+++ b/library/src/calculate_price.cpp
@@ -124,6 +124,30 @@ CalcState CALCULATE_LIBRARY_API compute_price_for_parking_ticket(
         }
         price->units = cost;
         price->netto = cost;
+    } else {
+        return calcState.set(CalcState::State::INVALID_START_DATE);
+    }
+
+    return calcState.set(CalcState::State::SUCCESS);
+}
+
+CalcState CALCULATE_LIBRARY_API compute_duration_for_parking_ticket(
+                                            parking_tariff_t *tariff,
+                                            time_t start_parking_time,
+                                            double price,
+                                            QString &duration) {
+    CalcState calcState;
+    QDate const d(1970, 1, 1);
+    QTime const t(0, 0, 0);
+    QDateTime start(d, t, Qt::UTC);
+    start = start.toLocalTime().addSecs(start_parking_time * 60);
+    if (start.isValid()) {
+        QString cs = start.toString(Qt::ISODate);
+        duration = calculator.GetDurationFromCost(tariff, PaymentOption::Option1,
+                                                  cs.toLocal8Bit().constData(),
+                                                  price, false, true).c_str();
+    } else {
+        return calcState.set(CalcState::State::INVALID_START_DATE);
     }
 
     return calcState.set(CalcState::State::SUCCESS);