function getHour(){
var currentTime = new Date();
return (currentTime.getHours());
}

function doGreeting(){
var hour=getHour();
var greeting="";

if (hour>0 && hour<12) greeting="Bom Dia";
else if(hour>=12 && hour<18) greeting="Boa Tarde";
else greeting="Boa Noite";

return (greeting+",");
}

document.write(doGreeting());