Simple adding expression

Try this:

st = time - thisLayer.inPoint; // Starts counting at layer in point 
t = st * .2;// divides time into 5 second intervals
value + Math.floor(t)

The first keyframe (0) is at the layer in point (0:00:01:15), the second is at (0:00:15:01 - 5). The slider value jumps exactly by 1 every five seconds after the in point.

The slider will increase in value by 1 every 5-seconds after the layer in point. It does not matter what value you assign to the slider. You can even keyframe the slider, so it slowly moves from 1 to 10, but every 5-seconds, the current value will increase by 1.