if all else fails in 2.0 a string usually works. Using the format and DATETIME should work though. I just had a similar issue in some code I was working on where no matter what the datatype was for the field, it would NOT accept it. I tried everything and no go so wound up not setting that field which luckily i didn't need. Contacting support may help but others have mentioned deleting and recreating the field may help as well.