Código HTML:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script language="javascript">
function setListB(a)
{
var inicio = a.selectedIndex;
var cant = a.length;
var b = document.formulario.horaB;
b.length = cant - inicio;
for (i = inicio+1; i < cant ; i++)
{
b.options[i-inicio].value = a.options[i].value;
b.options[i-inicio].text = a.options[i].text;
}
if (b.length == 1) b.options[0].text = "22:00";
}
</script>
</head>
<body>
<form name="formulario">
<select name="horaA" size="1" onChange="setListB(this)">
<option selected></option>
<option value="7:00">7:00</option>
<option value="7:20">7:20</option>
<option value="8:00">8:00</option>
<option value="8:10">8:10</option>
<option value="9:00">9:00</option>
<option value="9:10">9:10</option>
<option value="10:00">10:00</option>
<option value="10:10">10:10</option>
<option value="11:00">11:00</option>
<option value="11:50">11:50</option>
<option value="12:00">12:00</option>
<option value="12:40">12:40</option>
<option value="13:00">13:00</option>
<option value="14:00">14:00</option>
<option value="15:00">15:00</option>
<option value="15:20">15:20</option>
<option value="16:00">16:00</option>
<option value="16:20">16:20</option>
<option value="17:00">17:00</option>
<option value="17:20">17:20</option>
<option value="18:00">18:00</option>
<option value="18:10">18:10</option>
<option value="19:00">19:00</option>
<option value="20:00">20:00</option>
<option value="20:50">20:50</option>
<option value="21:00">21:00</option>
</select>
<select name="horaB">
</select>
</form>
</body>
</html> Servido, amigo!