Movable Type 2.21 has now been released with a very important bug fix to the MySQL schema.
The bug was that the created_on columns were timestamp fields, which means that MySQL updates them when an explicit value is not provided for them--this modifies the Authored On dates of any entries you modify. This bug will not occur if you are using MT itself, but only if you try to issue an insert or update command from the mysql client, phpMyAdmin, or another client tool. Thanks to shanson for finding this bug.
Version 2.21 has an updated schema that fixes the problem. So:
If you have not yet upgraded to 2.21 or installed Movable Type:
You can just install the system as normal, because the fixed schema has been integrated into the distribution.
If you have already upgraded, AND you are using the MySQL support:
1. Upgrade to 2.21 per the usual upgrade instructions.
2. Download this additional file and unpack it. It contains a CGI script called mt-upgrade221.cgi.
3. Upload mt-upgrade221.cgi to your webserver in ASCII mode into the directory containing mt.cgi.
4. CHMOD mt-upgrade221.cgi to 755
5. Run mt-upgrade221.cgi from your web browser. It should list the "alter table" statements that it is performing, then tell you that "all went well". If so, you're upgraded successfully, and you can delete mt-upgrade221.cgi from your server.
If you have already upgraded, and you are not using the MySQL support:
It is not urgent that you upgrade to 2.21, although there are some other small bug fixes in this release. Note: if you're not using MySQL, you don't have to run mt-dbupgrade221.cgi.