style.css

.hour {
  ...
  animation: rotateHourHand calc(12 * 60 * 60s) linear infinite;
  animation-delay: calc(calc(var(--start-minutes) * -60 * 1s) + calc(var(--start-seconds) * -1 * 1s));
}
@keyframes rotateHourHand {
  from {
    transform: translate(20px, 20px) rotate(calc(var(--start-hours) * 30deg));
  }
  to {
    transform: translate(20px, 20px) rotate(calc(var(--start-hours) * 30deg + 360deg));
  }
}