If you upgrade or install wordpress and find that you are no more able to add new posts or pages (no PUBLISH button), but are able to edit and update old posts here is what you do.
Update these two files : -
Replace '0000-00-00 00:00:00' with '1000-01-01 00:00:00'
replace every '0000-00-00 00:00:00' with '1000-01-01 00:00:00'
You will get your PUBLISH button again for all your new posts.