code: public static final long convert(long nanoTime)
      {
          final long timestampBase[] = TIMESTAMP_BASE;
          return timestampBase[0] + TimeUnit.NANOSECONDS.toMillis(nanoTime - timestampBase[1]);
      }
docs: /*
  System.currentTimeMillis() is 25 nanoseconds. This is 2 nanoseconds (maybe) according to JMH.
  Faster than calling both currentTimeMillis() and nanoTime().
  
  There is also the issue of how scalable nanoTime() and currentTimeMillis() are which is a moving target.
  
  These timestamps don't order with System.currentTimeMillis() because currentTimeMillis() can tick over
  before this one does. I have seen it behind by as much as 2 milliseconds.