Finding source file date/time from XSLT

I am current using Ant 1.6.5, Saxon 8.6 and Xerces 2.7.1 to transform a
large set of XHTML files (adding header, footer, menu bar etc.).
Additionally, I would like to add a time stamp of the source file in the
transformed file (i.e. not the time the file was transformed, but the last
time it was edited).

When browsing the XSLTProcess.java from the Ant distribution, it looks as
if the input file is send to the XSLT processor as a java.io.File object.
So as far as I can see, it should be possible to write a Saxon
Extension function, which can return the date/time of the processed XML
file - please, correct me if I am wrong.

Since I am far from a Java Whiz, I hope that somebody can provide me with
a hint or two on how to implement such an extension function.

Regards,

Jens