The lastmod field in sitemaps is subject to a lot of trouble for most people who are trying to build their own dynamic sitemaps. The main issue is that google only accepts a single type of timestamp and if it is now 100% correct it will throw a load of errors telling you the date is incorrect. I had a lot of difficulty finding the correct format to use for the timestamp. Quite often the solution would result in a date set to 1970, which of course was very incorrect. The solution below is how i was able to correctly use a MySQL timestamp in a Google sitemap for the lastmod field.
$datetime = new DateTime($row['Timestamp']);
$lastmod = $datetime->format('Y-m-d\TH:i:sP');
This will generate the correct output provided the input was first a valid SQL timestamp. One thing to be aware of and it is something that caused me to waste quite a lot of time. Even though you have updated the sitemap and the date is parsing correctly, Googles webmaster tools might still show the same error and will continue to show the error for a few minutes (even as far as an hour). As long as you are confident that the format is what it should be, wait a while and the webmaster tools page will eventually update correctly.