Original: Every editor's default signature (defined by MediaWiki:Signature) will display when ~~~~ is typed.
Simplified: To sign a post, type four tildes, like this: ~~~~. To type the username only, type 3 tildes:~~~, and this would appear: Example. To type only the date, type five tildes:~~~~~, and this will appear:00:00 01 January 1000 (UTC).