A simple example in PHP:
<html>
<body>
<span><? echo date('Y') ?></span>
</body>
</html>
Note: PHP has to be integrated with a webserver.
A slightly less simple example in JavaScript:
<html>
<script>
function currentYear() {
y = new Date();
document.getElementById("year").innerHTML = y.getFullYear();
}
</script>
<body onload="currentYear()">
<span id="year"></span>
</body>
</html>
Should work with any JS supporting browser