How can I restore author and post information for nodes? When I create a content type, I disabled showing "author name" and "posting date"; then, I created nearly 100 nodes from that content type.

Now I want to show the posting date in the node content.

When I edit the node, and enable the posting dates to be shown, everything is OK; when I edit the content type, and enable the posting date, I can't see it in node page.

It is hard to edit all nodes to show the posting date. Is there a better way?
Regarding both date and author info:
If you are not comfortable updating the Post date directly in the database, then I suggest you take a look at Views bulk operations, which is capable of performing mass manipulation of your sites content.

If the data doesn't show up as expected, it's always a good idea to switch to stark, to see if it's a theme issue.