Howto get lastmodified date of original XML file?

["Linden H van der (MI)" <H....@MI.unimaas.nl>]

Guys,

It might be staring me right in the face, but I don't see it. 

I'm building a simple website with Cocoon using XML files as starting
point. On the resulting HTML page I want to show the lastModified date,
but my script that uses document.lastModified uses time of Cocoon
generation.

What should I do to get the lastModified date of the original XML file?

Thanks.

Bye, Helma