diff --git a/library/src/calculator_functions.cpp b/library/src/calculator_functions.cpp
index 7aec757..b8501a1 100644
--- a/library/src/calculator_functions.cpp
+++ b/library/src/calculator_functions.cpp
@@ -266,11 +266,19 @@ std::string Calculator::GetDurationFromCost(Configuration* cfg, uint8_t payment_
 	cout << "Total minutes: " << (int)ret_val << endl;
 	if (ret_val <= 0) return "PARKING NOT ALLOWED";
 
-	struct tm valid_until_datetime = *localtime(&valid_until_datetime_t);
-	LOG_DEBUG("Ticket is valid until ", asctime(&valid_until_datetime));
+    struct tm valid_until_datetime = *localtime(&valid_until_datetime_t);
+    // TEST: no daylight saving time
+    // valid_until_datetime.tm_isdst = 0;
+
+    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;
-	return asctime(&valid_until_datetime);
+    return std::string(buf);
 }
 
 ///////////////////////////////////////