Thank you very much for your answers.
First excuse me, I was not very clear. The idea is to have it running for a little more than 70 years, but when it's off i just want it to remember when it stops and start at the last knew count when i turn it back on.
So is real time clock necessary ? i guess i'll need a independent battery device anyway.
The next steps are :
1- getting the 10 diplays, the 2 max7219, the memory device, and the arduino mounted correctly together. I'll try to do it by myself but if you can give me any advice or link or help, it will be welcome.
2- put the right countdown program in the arduino.
3- link the two sensors that can pause the countdown when they are both activated.
I'll share my progessions with you
If i make any mistake in my steps feel free to warn me, i only wish to learn.