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.1
When --backup-converted (-K) is specified in conjunction with -N, server file X is compared to local file X.orig, if extant, rather than being compared to local file X, which will always differ if it's been converted by --convert-links (-k).
Arguably, http time-stamping should be implemented using the
If-Modified-Since request.
[1] As an additional check, Wget will look at the
Content-Length header, and compare the sizes; if they are not the
same, the remote file will be downloaded no matter what the time-stamp
says.