output: #fixed code
                                  @Override
      public void format(final LogEvent event, final StringBuilder toAppendTo) {
          final long timestamp = event.getTimeMillis();
          toAppendTo.append(timestamp - startTime);
      }
input: #vulnerable code 
                                  @Override
      public void format(final LogEvent event, final StringBuilder toAppendTo) {
          final long timestamp = event.getTimeMillis();
  
          synchronized (this) {
              if (timestamp != lastTimestamp) {
                  lastTimestamp = timestamp;
                  relative = Long.toString(timestamp - startTime);
              }
          }
          toAppendTo.append(relative);
      } 
                              #location 11 
                              #vulnerability type THREAD_SAFETY_VIOLATION