gistfile1.js

var h = new Date().getHours();
var slots = $H({
  morning: $R(4,11).toArray(),
  afternoon: $R(12,16).toArray(),
  evening: $R(17,23).toArray().concat($R(0,3).toArray())
});
slots.each(function(time){
  if(time.value.include(h)){
    if(!!$('video').readAttribute('poster')){
      $('video').writeAttribute('poster', 
      $('video').readAttribute('poster').
      replace(/morning/, time.key));
    }
    if(!!$('video').readAttribute('src')){
      $('video').writeAttribute('src', 
      $('video').readAttribute('src').
      replace(/morning/, time.key));
    }
    $('video').select('source').each(function(elm){
      elm.src = elm.src.replace(/morning/, time.key);
    });
  }
});