This command line assumes that “${url}” is the URL of the web resource:

curl -s -I "${url}" | grep 'Last-Modified:' | cut -c 16- | date -f - +'%s'

It can be useful to check the freshness of a download URL before a GET request.

You could compare the result to the last-modified timestamp of a local file and only download the remote file if it is newer than the existing local one.