timer.js

import React from 'karet';
import Kefir from 'kefir';

const tick = Kefir.interval(1000, 1);

const Timer = ({ start = 0 }) =>
  <div>
    Seconds: {tick.scan((total, i) => total + i, start)}
  </div>;

timer.tag

<timer>

  <p>Seconds Elapsed: { time }</p>

  <script>
    this.time = opts.start || 0

    tick() {
      this.update({ time: ++this.time })
    }

    var timer = setInterval(this.tick, 1000)

    this.on('unmount', function() {
      clearInterval(timer)
    })
  </script>

</timer>