I made a thing:
In case someone other than me was confused by positioning clips with extend script.
In the GUI, I select the part of a video that I want to use by editing it in the footage panel. Essentially, I am trying to make frame n appear at time n in the timeline.
But, the javascript API doesn't do this and as far as I can the documentation does not describe how it works. It refers to the start time and the in point
An analogy that I find helpful is that the composition is like a slide rule (an ancient device your grandmother used. look it up).
Footage is referenced from a layer. Each layer is within a track that is fixed with respect to the composition's timeline and layers contain items.
Using extendscript you effectively push the clip along the track to make what is called the "in point" line up with the place in the timeline where you want it to occur.
As you can see, to make frame 3 occur at frame 2 in the composition, you specify that the in point is 0 and the start time is -1. That was very confusing for me.
So, start time is where in the layer the start of available footage is. The in point is where in the layer the start of video to be rendered will occur.