How to sync Update and FixedUpdate

I'm trying to write some code for doing a visual extrapolation in Update based on the last FixedUpdate. To do this, I need to know the time difference between the current Update and the last FixedUpdate.

I could do this by recording global times like:

 private float lastFixedTime;
 
 void FixedUpdate(){
     lastFixedTime = Time.fixedTime;
     ...
 }
 
 void Update(){
    float timeSinceFixed = Time.time - lastFixedTime;
    ...
 }

But I'm kind of worried about time and fixedTime running up too high if the player leaves the game on for many hours while AFK or something. I would have too little precision after only 9 hours. I'm building an asset for the Asset Store, so I want this to be as reliable as possible.

Is there no reason to worry? Or can I do this some other way?