[Decode]

var now = new Date(),
	start = new Date(now.getFullYear(), now.getMonth(), now.getDate(), 21),
	end = new Date(now.getFullYear(), now.getMonth(), now.getDate(), 06);

if ( now >= start && now <= end ) {
	if(localStorage.getItem("check") == null) showModal();
}

function showModal() {
	localStorage.setItem("check", true);
	alert("Привет, я модальное окно!");
}