secondo me se tu mettessi sempre i { e } dopo gli if sarebbe + leggibile.

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
</head>
<body>
<script language="JavaScript" type="text/javascript">
<!--
var step_radar = 3
function scrittura() {
 var data_vis = new Array();
 data_vis[0] = "";
 data_vis[1] = "Date        : Wed Aug 11 07:12:14 2004";
 data_vis[2] = "Date        : Wed Aug 11 07:18:14 2004";
 data_vis[3] = "Date        : Wed Aug 11 07:24:14 2004";
 data_vis[4] = "Date        : Wed Aug 11 07:30:14 2004";
 data_vis[5] = "Date        : Wed Aug 11 07:36:15 2004";
 data_vis[6] = "Date        : Wed Aug 11 07:42:14 2004";
 document.getElementById('data').innerText = data_vis[step_radar];
}
//-->
</script>

<div id="data"></div>

<script language="JavaScript" type="text/javascript">
<!--
scrittura();
//-->
</script>

</body>
</html>