odometer:
Let's see if I read the situation right:
You want a timer. But you can't bother to learn to code, so you pull up someone else's timer project from a foreign language Web site and expect us to adapt it to your needs. That, and it is not even clear how this timer project is supposed to work: I tried the Arduino Uno version you gave us in post #1 , and I couldn't even get the timer to run.
This happens often here. People who can't bother to learn to code give us code that they don't understand, and can't be bothered to try to understand, that they expect us to adapt to their needs, thinking it will be an easy fix, even when the only sensible solution would be to throw everything away and start from scratch. That, and if you expect code to be written for you, for free, you will probably be disappointed.
As far as Arduino projects go, a timer is not a hard one to code. You might, however, find it a bit of a challenge to make this timer run at the same time as your 7-segment display countdown is running. I am sure that that is the next thing you will want: you will want to make both this timer and the 7-segment display countdown run at the same time, and you will find this to be harder than you anticipated.
Combining two projects to run on the same machine, at the same time, requires that you know what you are doing. If you cannot be bothered to take the time to learn, then that is what the Jobs and Paid Consultancy forum category is for. Here is a link to that category: Jobs and Paid Consultancy - Arduino Forum
Sorry, my mistake, I didn't know there was this category. I did not intend to insult anyone or not pay for the work.