Even though the value stored in millis() rolls over after a certain length of time, you're only checking to see if it's increased by 1000... so it doesn't matter! It's not as if your code will drop dead as soon as it rolls over, it'll just start ticking over from 0 again and your other variables will stay how they were.
To print more than one number, just repeat that statement! The 7219 will latch it's digits so they won't change until you write something else to that location. I did have some example code somewhere, but I can't find it 
Sorry.