What's going on here?

import com.google.common.base.Stopwatch;
import java.time.Duration;

public class Main {

    public static int WIDTH, HEIGHT;

    private static Stopwatch runtime;
    private static Duration lastElapsed = Duration.ZERO;

    private static Duration duration() {
        Duration temp = lastElapsed;
        return (lastElapsed = runtime.elapsed()).minus(temp);
    }

    public static void main(String[] args) {

        StringBuilder output = new StringBuilder();
        output.append("Output: \n");
        runtime = Stopwatch.createStarted();
        for (int i = 0; i < 100; i++) {
            if(i >= 18) output.append((double)duration().toNanos() / 1000 + "\n");
            else output.append("Initializing...\n");
        }
        System.out.println(output);

    }
}

/* stdout:
Output:
Initializing...
...
Initializing...
8.673
2403.109
22.5
12.479
10.647
9.386
9.294
8.77
9.662
8.697
9.082
8.619
8.777
8.556
8.75
8.928
8.739
8.462
9.751
9.632
...
*/