.... when you can remember what a slide rule was/is (sometimes called a Slipstick). What is worse, is they I could get advanced results out of that quicker than most people these days can turn a computer on. We used to have competitions to see who was the quickest slipsticker). I was pretty quick, but there were a couple of colleagues who were just amazing.

I have to admit to being pretty good with this in my day http://en.wikipedia.org/wiki/Curta_calculator

I was pretty good with one of these
http://www.vintagecalculators.com/html/muldivo_mentor.html