It has been a while since I flew ADD to JNB, but I do not remember anyone checking my Yellow Fever card in ADD, but then again, I was in transit. I would say the health inspection in JNB are the ones who really make a fuss, but they do this for a few flights from "high risk" areas, and I am not sure Ethiopia is considered one of those. More often than not checking in JNB is based on where you travels originated, so if you were from outside Africa, they will lily not ask for it. Sorry, there is no straight answer to your question, but as others have posted, they may only be interested in the page having a vaccination stamp, and not really the date. All the best, and do keep us posted.