Time-stamping in HTTP is implemented by checking of the
Last-Modified header. If you wish to retrieve the file
`foo.html' through HTTP, Wget will check whether
`foo.html' exists locally. If it doesn't, `foo.html' will be
retrieved unconditionally.
If the file does exist locally, Wget will first check its local
time-stamp (similar to the way ls -l checks it), and then send a
HEAD request to the remote server, demanding the information on
the remote file.
The Last-Modified header is examined to find which file was
modified more recently (which makes it "newer"). If the remote file
is newer, it will be downloaded; if it is older, Wget will give
up.(2)
Arguably, HTTP time-stamping should be implemented using the
If-Modified-Since request.
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