var Digital=new Date()
var hours=Digital.getHours()

if (hours>=7&&hours<=11) 
document.write('Bom dia')
else if (hours>=13&&hours<=19) 
document.write('Boa tarde')
else if (hours>=19&&hours<=6) 
document.write('Boa noite')
else if (hours>=21&&hours<=11) 
document.write('Boa noite')
else 
document.write('Boa noite')