Animation from random position to specific

Hi all,

I want to distribute multiple layers in Z axis and on specific time to animate them at '0' point. I've managed to distribute them randomly and place them at '0' point in a given time, but with no animation.. it just 'jumps' to '0' point. This is what i've used:

seedRandom(18, true);
z= random (400,-400);

if (time > 12)  z=0;
[z]

I know i'm missing something.. any hints?

Thanks