That is entirely up to the server. For the usual file objects, it is just
the file system last-mod date -- the server should be aware of the limitations
of the OS under which it runs. For objects with includes, it may be the
most recent of the set of last-mod dates of its component parts. For
database gateways, it may be the last-update timestamp of the record.
For virtual objects, it may be the last time its internal state changed.
In any case, the HTTP client should only know (and care) about the result --
whatever gets stuck in the Last-modified: header -- and not worry about
how it was obtained.
However, under no circumstances should it ever come from the HTML header.
That mechanism was never intended to provide values for metainformation
that can be (and is) more readily obtained elsewhere.
.....Roy Fielding ICS Grad Student, University of California, Irvine USA
<fielding@ics.uci.edu>
<URL:http://www.ics.uci.edu/dir/grad/Software/fielding>