TIMESTAMP and DATETIME fields may be automatically updated, at least on MySQL, see http://dev.mysql.com/doc/refman/5.6/en/timestamp-initialization.html . However, this feature isn't available / doesn't work in Adminer. TIMESTAMP has two options (ON UPDATE) and CURRENT_TIMESTAMP and I guess it was meant to be only one option, here neither works. DATETIME has no options, it should have he same as TIMESTAMP.
You can set a default value for timestamp in the field Default values. I've added the same option for datetime column.
The default value seemed to work, I was rather talking about the ON UPDATE CURRENT_TIMESTAMP property. I was unable to make it work with Adminer, so I set both with a SQL command. The table summary also doesn't show the ON UPDATE property.