fixed sonar bugs

sharepoint/api/src/main/java/de/muehlencord/shared/sharepoint/api/SPHelper.java

@@ -1,7 +1,6 @@
 package de.muehlencord.shared.sharepoint.api;
 
 import java.text.SimpleDateFormat;
-import java.time.format.DateTimeFormatter;
 import java.util.Date;
 
 /**
@@ -18,7 +17,13 @@ public class SPHelper {
         if (d == null) {
             return "";
         }
-        return dateOnlyFormat.format(d);
+
+        String result;
+        synchronized (dateOnlyFormat) {
+            result = dateOnlyFormat.format(d);
+        }
+        return result;
+
     }
 
     /* -- Format for DateTime Field -- */
@@ -26,8 +31,12 @@ public class SPHelper {
         if (d == null) {
             return "";
         }
-
-        return dateTimeFormat.format(d);
+        String result;
+        
+        synchronized (dateTimeFormat) {
+            result = dateTimeFormat.format(d);
+        }
+        return result;
     }
 
 }