Is -System.nanoTime() + System.nanoTime() guaranteed to be >= 0?

Hi all I have a piece of code which looks like this:

public class Test {
    public static void main(String args[]) {
        long a = System.currentTimeMillis(); // line 1
        long b = System.currentTimeMillis(); // line 2
        assert b - a >= 0;

        long y = System.nanoTime(); // line 5
        long z = System.nanoTime(); // line 6
    }
}

So IERS stated that the next leap second is to occur immediately after 30^th June 2012 11:59.9.

I was wondering if I'm right to say that if line 1 is run at 0.9 seconds after 30^th June 2012 11:59.9 turns 1^st July 2012 00:00.0,

And line 2 is run at 0.1 second after line 1,

The result of b - a could be negative ? (-900 milliseconds)

If that's the case, is it true that if line 5 is run at 0.9 seconds after 30^th June 2012 11:59.9 turns 1^st July 2012 00:00.0,

And line 6 is run at 0.1 second after line 5,

The result of z - y could be negative ? (-900,000,000 nanoseconds?)