also your code needs to be somewhat like this. this file should be .shtml and not .html
and the reason why it is not interpreted by the ssi servlet.
<!-- the modified code is -->
<!DOCTYPE html>
<html lang="de">
<head>
<meta charset="UTF-8">
<title>Using SSI</title>
</head>
<body>
<!-- this becomes header.html and not header.shtml -->
<!--#include virtual="header.html" -->
<!--#echo var="DATE_LOCAL" -->
test page.
</body>
</html>