Attempts to cache the sent entity by its last modification date.
If the supplied argument is greater than 0, it is handled as timestamp
and will be sent as date of last modification. If it is 0 or omitted,
the current time will be sent as
If it's negative, it is handled as expiration time in seconds, which means
that if the requested last modification date is not between the calculated timespan,
Last-Modified header is updated and the actual body will be sent.
Caching for 5 seconds.