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Код

<!DOCTYPE HTML>
 
<html>
 
<head>
  <title>Untitled</title>
  <meta charset="utf-8">
  <style type="text/css">
 
  </style>
   <script>
    function cost(run, end) {
    var lunch = [(new Date()).setHours(12, 0, 0, 0),(new Date()).setHours(12, 45, 0, 0)]
    run =  (new Date()).setHours(run[0], run[1],0,0);
    end =  (new Date()).setHours(end[0], end[1],0,0);
    run > lunch[0] && (lunch[0]=run);
    end < lunch[1] && (lunch[1]=end);
    if(run > lunch[1]||end < lunch[0]||run > end) return 0;
    return (lunch[1] - lunch[0])/60000 ;
};
    document.addEventListener("input", function(event) {
    var el = event.target;
    if (el.classList.contains('time')) {
      var inp =  document.querySelectorAll('.time');
      if (inp[0].value.length != 5 || inp[1].value.length != 5) return ;
         var run =  inp[0].value.split(':'),
          end =  inp[1].value.split(':');
          inp[2].value = cost(run, end)
 
    }
});
 
  </script>
</head>
 
<body>
<input type="text" class="time">
<input type="text" class="time">
<input type="text" class="time">
</body>
 
</html>