He did actually. While being beaten. It's in the article. not/s!

Chisanbop or chisenbop (from Korean chi (ji) finger + sanpŏp (sanbeop) calculation 지산법/指算法), sometimes called Fingermath, is a finger counting method used to perform basic mathematical operations.

https://en.wikipedia.org/wiki/Chisanbop

You might be already doing this. If you strum your fingers of your right hand by pressing your index, middle, ring, and pinky to your desktop, and then do the same thing again starting with your thumb, you've just counted from 0 to 9. Do the same on your left hand and you've gone from 00 to 90. It's really easy to do simple math this way by counting on your fingers.

For stimming purposes, you might just start by counting up or counting down, then maybe counting up by twos or counting down by threes.

This is the approach that I've known for many decades now. I've seen YouTube videos of kids doing amazing fast calculations like multiplying large numbers using what looks like a different method in that their hands are in the air. I'll leave it to you to Google the other approaches if this direction interests you.

Practicing touch typing.

I don't know how many times I've absent-mindedly "strummed" my fingers by tapping out "This is a test of the emergency broadcast system. This is only a test. In the event of a real emergency...", a TV memory from my childhood.

When I first learned touch typing, I did consciously practice this way. ASDF, JKL;. The quick brown fox jumps over the lazy dog.

Quarter after four is 4:15.

Quarter of five is 4:45. Also quarter to five and quarter til five.

I'm seeing other comments that suggest I might be wrong. Especially in regards to other languages.