How to do something 1 time in Update?

Question

C++ / C#

I want to make my game, for example, increase the difficulty every 2 minutes, but I could not find on the Internet how to work with time in this way, and therefore I wrote my code like this:

private void FixedUpdate() 
    {
        StartTime += 0.1f * Time.deltaTime;
        if (StartTime >= EndTime)
            spawnRate--;
    }

but it only works in Update, and therefore it happens not once, but a lot, how can I fix it?

Answer 1

Counting in FixedUpdate Time.deltaTimeyou get nonsense.

so it doesn't happen once, but many times

And who will install the next one EndTime?