I had this problem as well, after searching the internet to no avail I discovered what was causing it for me. I'm running Mediawiki 1.6.10 for reference, an older version for php compatibility reasons. Anyways the problem was that the character limit for the revision date in the MYSQL database was too small for the timestamp. Instead of being 14 characters limited it was set to 4 for some reason. In your SQL db for your wiki look for the table called yourdbprefix_revision in that table find the column rev_timestamp and check its datatype, if it's set to anything less then VarChar(14) change it and save the changes. Now when you edit a page the revision will show the correct date.