[xsl] Modification date/time of an XML file during its XSLT-processing - how to get?

[Blue Gecko <bluegecko@xxxxxxxxx>]

Hello folks,

I need to insert the modification date/time (timestamp) of a source XML 
file into the transformed contents, just like this:

<!-- XSLT output.
"lastModification" attribute comes from the timestamp of my source XML file.
-->
<foo lastModification="2005-10-04T13:20:00.000+01:00">
<items/>
</foo>

could anyone suggest me how to obtain such timestamp in the context of 
an XSLT processing?

Many thanks