diff --git a/library/src/calculator_functions.cpp b/library/src/calculator_functions.cpp
index b8501a1..c8eea44 100644
--- a/library/src/calculator_functions.cpp
+++ b/library/src/calculator_functions.cpp
@@ -10,7 +10,12 @@ double total_cost = 0.0f;
 bool overtime = false;
 
 /// <inheritdoc/>
-std::string Calculator::GetDurationFromCost(Configuration* cfg, uint8_t payment_option, char const* start_datetime, double price, bool nextDay, bool prepaid)
+std::string Calculator::GetDurationFromCost(Configuration* cfg,
+                                            uint8_t payment_option,
+                                            char const* start_datetime, // given in local time
+                                            double price,
+                                            bool nextDay,
+                                            bool prepaid)
 {
 	// Get current date time from input
 	struct tm current_datetime = Utilities::DateTimeToStructTm(start_datetime);
@@ -264,17 +269,31 @@ std::string Calculator::GetDurationFromCost(Configuration* cfg, uint8_t payment_
 	else ret_val = total_duration_min;
 
 	cout << "Total minutes: " << (int)ret_val << endl;
-	if (ret_val <= 0) return "PARKING NOT ALLOWED";
+    if (ret_val <= 0) return "PARKING NOT ALLOWED";
 
-    struct tm valid_until_datetime = *localtime(&valid_until_datetime_t);
-    // TEST: no daylight saving time
-    // valid_until_datetime.tm_isdst = 0;
+#if !defined(_SVID_SOURCE) && !defined(_XOPEN_SOURCE)
+    // needed for timezone-correction
+    #error "!defined(_SVID_SOURCE) && !defined(_XOPEN_SOURCE)"
+#else
+    // timezone correction: localtime() needs argument in UTC-timezone
+    // The global variable 'timezone' is set by tzset(), see
+    // https://linux.die.net/man/3/tzset, so I change of the timezone will
+    // also change the value of the variable 'timezone'.
+    valid_until_datetime_t += timezone;
+#endif
 
+    struct tm valid_until_datetime;
+    memset(&valid_until_datetime, 0x00, sizeof(valid_until_datetime));
+
+    if (!localtime_r(&valid_until_datetime_t, &valid_until_datetime)) {
+        return "LOCALTIME_R() ERROR";
+    }
+
+    // return in ISO-format: "%Y-%m-%dT%H:%M:%S"
     char buf[128];
     memset(buf, 0x00, sizeof(buf));
     strftime(buf, sizeof(buf)-1, "%Y-%m-%dT%H:%M:%S", &valid_until_datetime);
 
-    // LOG_DEBUG("Ticket is valid until ", asctime(&valid_until_datetime));
     LOG_DEBUG("Ticket is valid until ", buf);
 
 	total_duration_min = 0.0f;