How to get value of form field and past it to another form field

I am developing HR system, I want to calculate how many days off the employee can take.
In the HR table I have three attributes
1- start_date(when the employee starting to work in the company).
2- leavestaken(how many days off the employee took before).
3- leaves(the number of days available for the employee to take off) which equal to=(months between current date and start_date) * 2.5(beacuse each month give 2.5 days off) - leavestaken

third attribute is automated and not from the user, how I can get the first and second attributive values from field forms, use them in the calculation and save the result in the leaves attribute?

I sloved it,
my solution is

if ($model->load(Yii::$app->request->post()) ) {

       $date = date('m/d/Y');

     

     $datetime1 = date_create($date); 

     $datetime2 = date_create($model->start_date); 

       

     // calculates the difference between DateTime objects 

     $interval = $datetime2->diff($datetime1);

     $n_month= $interval->m + 12*$interval->y;

     $model->leaves = ceil($n_month * 2.5) - $model->leavestaken;

                

        $model->save();