The usage of time-stamping is simple. Say you would like to download a file so that it keeps its date of modification.
wget -S http://www.gnu.ai.mit.edu/
A simple ls -l shows that the time stamp on the local file equals
the state of the Last-Modified header, as returned by the server.
As you can see, the time-stamping info is preserved locally, even
without `-N'.
Several days later, you would like Wget to check if the remote file has changed, and download it if it has.
wget -N http://www.gnu.ai.mit.edu/
Wget will ask the server for the last-modified date. If the local file is newer, the remote file will not be re-fetched. However, if the remote file is more recent, Wget will proceed fetching it normally.
The same goes for FTP. For example:
wget ftp://ftp.ifi.uio.no/pub/emacs/gnus/*
ls will show that the timestamps are set according to the state
on the remote server. Reissuing the command with `-N' will make
Wget re-fetch only the files that have been modified.
In both HTTP and FTP retrieval Wget will time-stamp the local
file correctly (with or without `-N') if it gets the stamps,
i.e. gets the directory listing for FTP or the Last-Modified
header for HTTP.
If you wished to mirror the GNU archive every week, you would use the following command every week:
wget --timestamping -r ftp://prep.ai.mit.edu/pub/gnu/
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