I think the cleanest way to do this is via an automation and a script

There are probably alternatives to do it without a script, but they'd be very complicated, time consuming to set up, and fragile

Thanks for your answer, but I have no experience with script.
Can you help me, or tell me how to begin, maybe some videos tutorial, or other support?

You could try CodeAcademy's JavaScript course if you'd like. Without any prior experience this might be pretty difficult though

May I know if you're on a Pro account? Asking us only Pro accounts can run scripts via an automation.  If you are I can see if I can put something together for you

Yes I am on Pro Account.

Thanks

Here, this should do what you're looking for I think

Hello, thanks to take to answer my question on this january 1st.
I can't check that today, but for sure first thing tomorrow I will try your script.
Happy New year

Hello Adam, I tested it and it work exactly the way I was looking for. The only thing, it is a little bit slow, do you have an idea to speedup the process?
Thanks again!

Hello, your solution work fine.
But I had to changed my structure, indeed the finish information is from a lookup field, I changed the variable Finish to Lookupfiled.value, but now the script do not work.
I hope you can hep me again
Thanks

> The only thing, it is a little bit slow, do you have an idea to speedup the process?
I don't think so I'm afraid; I believe that's just how long automations take to trigger

> I hope you can hep me again
Could you post a screenshot of your new base structure please? 

Hello, I hope this is clear, if not let me know
Thanks again to help me

Yeap sure, check out this base

I had tried to use directly the linked field, but I was missing the function .toString()
Thanks again form your help and your time