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

How to convert System.currentTimeMillis(); to seconds?

long start6=System.currentTimeMillis();
System.out.println(counter.countPrimes(100000000)+" for "+start6);

The console shows me 5761455 for 1307816001290. I can't read how many seconds that is.

Any help?