Adding a 'date last modified' feature to pages

Hi guys, just wondering if you could help me out with adding a date last modified feature to pages in enfold.

I used a code block and added this code to it –

<SCRIPT LANGUAGE=”JavaScript”>
update = new Date(document.lastModified)
theMonth = update.getMonth() + 1
theDate = update.getDate()
theYear = update.getFullYear()
document.writeln(“<H6>Date last modified: ” + theDate + “/” + theMonth + “/” + theYear + “</H6>”)
</SCRIPT>

Sadly, it only shows the current date, for the past two days I’ve left that page alone and each day it’s changed.

Hope you can help.

Thanks,
Matt