How to convert nanoseconds to seconds using the TimeUnit enum?

Well, you could just divide by 1,000,000,000:

long elapsedTime = end - start;
double seconds = (double)elapsedTime / 1_000_000_000.0;

If you use TimeUnit to convert, you'll get your result as a long, so you'll lose decimal precision but maintain whole number precision.

TimeUnit Enum

The following expression uses the TimeUnit enum (Java 5 and later) to convert from nanoseconds to seconds:

TimeUnit.SECONDS.convert(elapsedTime, TimeUnit.NANOSECONDS)