How I can include the modification date of a webpage that might include
other files without having to enter the date myself? I'm interested in
including the latest modification date of the file and any file that
it includes.  I'm using Server-Side Includes (SSI) on my website and I
currently use
  
<!--#config timefmt="%A %B %d, %Y" -->

This file last modified <!--#flastmod file="ssi.shtml" -->

but this code returns the modification date of one file.  How can I
specify the latest modification date of several files?

You could write a script to get the date for all the concerned files;
put them into a matrix or DB table; then sort the results by year,
hour, minute, etc; and return the most recent value.