Update. s2member support responded with a bit of clarification: “The [s2Drip /] shortcode bases its calculations on Capability Access Times; i.e., when access was actually granted and/or purchased by the customer.”
Past that though, figuring out where / how to change that CAP for a specific user … I was pretty much told to go pound sand.
Would love to figure this out, still hoping for a bit of help from those smarter than I. ??