interview confusion

hi all!

these have sort of been answered so apologies I'm asking again but I just want to make sure I am super clear before i book.

  1. can i book the canadian portion of my nexus at an airport even if i'm not flying that day?

  2. if i don't complete the US portion the very next time im in the US, is it invalid?

I'm hoping to book the canadian portion at the end of april and then am going to vegas mid-may but our flight lands at like 10pm and i assume the nexus office won't be open.

if it doesn't have to be done THAT flight I am going to nashville later in the month and assume that could work, but want to be sure! thanks all!