It has to delay for 15 seconds it will just scroll through each display at lighting speed and no one would see anything.
I understand that, and what I'm telling you is that if you delay(15000), you will not be able to do anything else at all during those 15 seconds. There are other methods of causing portions of your sketch to wait 15 seconds while the rest of the sketch continues to execute. Blink without delay is one of those methods. Look at it. Learn it. Use it.