[Kaviton]

var Vb = getTag( 'Vb') || 0;
var vb2 = Vb;
var t1 = (new Date).getTime();
var vb1 = Vb;
var t2 = (new Date).getTime();
var y = (vb2-vb1)/(t2-t1);
setTag( 'Vb1', y);