frontend/js/common/entity/timer_time_remaining.js

import durationToSeconds from '../datetime/duration_to_seconds.js';

export default function timerTimeRemaining(stateObj) {
  let timeRemaining = durationToSeconds(stateObj.attributes.remaining);

  if (stateObj.state === 'active') {
    const now = new Date();
    const madeActive = new Date(stateObj.last_changed);
    timeRemaining = Math.max(timeRemaining - ((now - madeActive) / 1000), 0);
  }

  return timeRemaining;
}