Issue

The setInterval gets called each time when mytime changes for rerendering (when you call the setMytime). And the number of setInterval calls grows exponentially. This would lead to a memory leak as well.

Solution

You should run it only once. You should use useEffect hook with an empty dependency array.

Try like this.

import { useEffect, useState } from "react";

function App() {
  const [mytime, setMytime] = useState(0);

  useEffect(() => {
    // create a interval and get the id
    const myInterval = setInterval(() => {
      setMytime((prevTime) => prevTime + 1);
    }, 1000);
    // clear out the interval using the id when unmounting the component
    return () => clearInterval(myInterval);
  }, []);

  return <div> {mytime} </div>;
}

export default App;