code: public static double msFromNanoDiff(final long end, final long start) {
      if (end < start) {
        throw new IllegalArgumentException("End (" + end + ") cannot be less "
            + "than start (" + start + ")");
      }
      return ((double) end - (double) start) / 1000000;
    }
docs: Calculates the difference between two values and returns the time in
  milliseconds as a double.
  @param end The end timestamp
  @param start The start timestamp
  @return The value in milliseconds
  @throws IllegalArgumentException if end is less than start
  @since 2.2