<body>
<script>
var Digital=new Date()
var hours=Digital.getHours()
if (hours>=6&&hours<=12)
document.write('Bom dia.')
else if (hours>=12&&hours<=18)
document.write('Boa tarde')
else if (hours>=18&&hours<=22)
document.write('Boa noite.')
else
document.write('Boa madrugada.')
</script>
</body>