How to get the "theoretical" time left for a person?

Hello,

I apologize in advance for this question, which will probably seem trivial to you.

I would like to know if it is possible (and if so how) to extract the "theoretical" time left for a person. By "theoretical" time I mean the total number of days a person works during the year minus the number of project/activity days the agent is already assigned to.

I hope my question is pretty much clear. Please feel free to contact me again so that I can clarify/reformulate my request if necessary.

Yours sincerely.

Loïc