This version passes the given lastModified date through the client provided http conditional headers If-Modified-Since and If-Unmodified-Since.
Notice the second precision so it may work wrong if there were few changes during the same second.
For better behaviour use etag instead
See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.28 and https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25
lastModified - of the current content, for example file’s last modified date