It's a basic calculator made by the youtube channel Web Dev Simplified. He did a youtube tutorial on it, which I followed along and basically made the same thing.
Both his example calculator and mine has the same problem, where if I pressed on any of the number buttons (for example '1') 17 or more times, the numbers entered after the 16th digit displays as a different number for some reason (17th digit becomes '2' and not '1').
It does this with any number entered. I don't understand why.
I attached a codepen link so that you can see both his code and the code output.
You will find the display output in this part of the HTML code:
<div class="output">
<div data-previous-operand class="previous-operand"></div>
<div data-current-operand class="current-operand"></div>
</div>