How to make a variable fall/rise over time?

I’m somewhat new with scripting. I’m scripting a jump script and I’m trying to make it so that whenever the player is in the air a variable goes up, and that variable would negatively affect their jump height. So I wrote

void Update ()
if (isgrounded && (test>0) == true)  ;
   {int test =(test - 1);}

    if (isgrounded == false);
   {int test =(test + 1);} 

I realize that even if that worked it would increase/decrease the variable per-frame, rather that per-second, I was going to try to fix that later, maybe by making it a float

if (isgrounded == false);
   {float test =(deltaTime*(test + 1));} 

any help you can give would be greatly appreciated

to increase your amount per second it should be like this.

float test = (test + (Time.deltaTime * 1));

you could also try invokerepeating