How to measure the a time-span in seconds using System.currentTimeMillis()?

Solution 1:

TimeUnit

Use the TimeUnit enum built into Java 5 and later.

long timeMillis = System.currentTimeMillis();
long timeSeconds = TimeUnit.MILLISECONDS.toSeconds(timeMillis);

Solution 2:

long start = System.currentTimeMillis();
counter.countPrimes(1000000);
long end = System.currentTimeMillis();

System.out.println("Took : " + ((end - start) / 1000));

UPDATE

An even more accurate solution would be:

final long start = System.nanoTime();
counter.countPrimes(1000000);
final long end = System.nanoTime();

System.out.println("Took: " + ((end - start) / 1000000) + "ms");
System.out.println("Took: " + (end - start)/ 1000000000 + " seconds");