Common mistake that prevents Google from using your sitemap.xml file

According to official specification from sitemaps.org
(here: http://sitemaps.org/protocol.php#lastmoddef )
the format of the lastmod element in sitemaps file should be in the W3C Datetime format
but the actual time can be omitted.

One common mistake people make when generating sitemap file is to make the lastmod value in a format like this: YYYY-DD-MM HH:MM:SS
This is a wrong format and will not be understood by Google.
The correct W3C timestamp format is quite ugly, and looks like this:
(example) 1997-07-16T19:20:30.45+01:00

The best advice I can give you is to remove the HH:MM:SS and only leave the date part of the datetime, like this for example: 2010-03-23

This format is understood by all search engines and will not cause any problems.