I suspect it is when you make the reservation. I've never borrowed points. I have just recently received a detailed member point accounting statement and was surprised to see so much activity on it. One time I had called daily at the eleven month window to extend a reservation. For each day, they charged the points and the next day credited them back and charged the new cumulative total. Another time I needed two points from Vero, along with lots from Boardwalk to make a reservation at HH. They deducted the Vero points and the BW points immediately, even though I was seven months from the date of stay. If they do this for current use year points, then I suspect they will do the same for borrowed points.
Cap