I know this is an old posting, but we just found that some of our page modified dates are null. May have happened during an upgrade or maybe during a move to a different server, who knows?
But anyway we found out the easiest solution (if you have SQL access to your site). We simply asked to find all the pages with a modified date of null. We got way more than we expected. So we wrote a procedure that updated the date whenever it found a null.
Of course, we ran the update against each portal separately so that we could put a real date in it, reflecting at least the date the page was created.
By the way, some of our date created fields were blank as well. We used another query to update those. Of course, they should have never been blank.
In all fairness to DNN, some of these sites are 4-6 years old and have been through upgrades all the way from DNN 3.XXX.
One last thing, to update your sitemap.aspx immediately, go to the admin menu and the sitemap option. There is a 'clear cache'. Do that and you can immediately update the sitemap. Of course the default cache is only one day, so everything would update the next day anyway.
Why is this important? Google will report sitempa errors if the date is invalid.
Just another of the many things to keep your eye on.
We're not posting our SQL code, unless there is a demand for it. Just want everyone to be cautious when manually changing stuff directly in SQL.