Your problem here is that you're mixing logic and presentation, so it's neither clever nor clear. I'm not 100% sure what you're trying to do (your explanation again mixes everything together making it hard to understand) but I think you want to do this:

- Transform the screen position to a week-based relative coordinate system.

- Convert that spatial coordinate into a time stamp represented as seconds since the epoch.

That's a minimum of two steps, two verbs and therefore two separate functions. Step 1 can be broken down further by introducing helper functions for inspecting the DOM, decoupling the representation of the UI elements from your actual screen-space coordinate system, as CSS isn't in a nicely-scriptable format.

Once you've done this, you'll discover the result to be only marginally longer, but highly reusable.