var hours = new Date;

t = hours.getHours();

var strArray = new Array("おはようございます！","こんにちは！","こんばんは！");
var messeage;

if(t < 5){
	messeage = strArray[2];
}else if(t < 12){
	messeage = strArray[0];
}else if(t < 18){
	messeage = strArray[1];
}else if(t < 24){
	messeage = strArray[2];
}

// 表示
document.write(messeage);