Your server will (almost) always return a last-modified date along with the content of your page. If the page also contains a valid date meta-tag, the GSiteCrawler will assume that the date in the meta-tag is the actual date of the content and use that for further processing (i.e. for generating the Google Sitemap file). A date meta-tag is valid if it can be parsed properly and if it results in a date older than "now" (current date + time) and newer then January 1st, 1990. If several date meta-tags are used, it will use the order of precedence listed above (e.g. if both 'date' and 'dc.date.modified' are specified and valid, it uses 'dc.date.modified').