--- a/stress-tester/src/main/java/com/passus/st/generator/DateTimeGenerator.java	Mon Jun 15 12:47:22 2020 +0200
+++ b/stress-tester/src/main/java/com/passus/st/generator/DateTimeGenerator.java	Mon Jun 15 13:30:16 2020 +0200
@@ -116,12 +116,17 @@
     @Override
     public Object generate() {
         long time = startTime + (long) (Math.random() * (endTime - startTime));
-        DateTime dateTime = time > 0 ? new DateTime(time) : new DateTime();
-        if (returnType == ReturnType.DATETIME) {
-            return dateTime;
+        if (time < 0) {
+            return null;
         }
 
-        return formatter.print(dateTime);
+        if (returnType == ReturnType.DATETIME) {
+            return new DateTime(time);
+        }
+
+        synchronized (this) {
+            return formatter.print(time);
+        }
     }
 
     public static final class DateTimeNodeDefCreator implements NodeDefinitionCreator {