mafap/client/src/components/timerCard.tsx

import { ago } from "../utils/ago";
import { useEffect, useState } from "react";

export default function TimerCard({ timer }) {
  const [since, setSince] = useState(ago(timer.start));

  useEffect(() => {
    let updateTimersInterval;
    const msTillNextSecond = 1000 - (timer.start.getTime() % 1000);

    setTimeout(() => {
      updateTimersInterval = setInterval(
        () => setSince(ago(timer.start)),
        1_000
      );
    }, msTillNextSecond);

    return () => clearInterval(updateTimersInterval);
  }, []);

  return (
    <h1>
      <code>{since}</code> since {timer.name}
    </h1>
  );
}