MySQL release notes contain the version number and the release date: http://dev.mysql.com/doc/refman/5.1/en/news-5-1-x.html
MariaDB release notes contain only version number but no date: http://askmonty.org/wiki/Manual:MariaDB_5.1_Release_Notes
There is also no date inside the Release Notes articles.

The historical release date is useful information in many ways, it should be recorded in the Release Notes and Changelog. Preferably both in the table of contents and the article itself (but probably not in the article title so it doesn't come into the URL.)

This should be fixed both for our Release Notes and Changelogs.
If possible, it should be fixed also for old releases since there are still not that many of them.