Base time used for node last-modified calculations, in seconds.
Chickadee uses the overall repository modification time (updated whenever a reindex is done) to determine when a node was last modified. Browsers with a vaild cached copy will receive a "304 Not Modified" response.
However, the value of last-modified will also be considered; the modification time is taken as the maximum of the two values. This is done because changes to program logic may change the rendering of the page even when the repository has not changed. For example, set last-modified to (current-seconds) to simply invalidate all pages when the server is restarted.
Defaults to 0.