expression for path

Hi,
I am following a tutorial. I applied some key frames on path (of a Stroke) and want to loop it. The teacher says, with path key frames you can’t use loop in expression, because it only works with numerical values. Tough, One expression you can use to loop path key frames if the key frame starts in ‘0’ (zero). And another for, if the key frames do not start at zero.
My question is-
What he refers as zero?
The first keyframe of the loop? Or the first keyframe for entire composition?
Here my composition duration is six seconds. But he started the loop from 2nd to 4th second.
What is zero frame in such scenario?
The first 0.00f or 2.00f?
Thank you.