The "Your signature with timestamp" button on the edit toolbar should generate ~~~~, not --~~~~. The extra dashes make it inconsistent with the character map below the edit box which uses ~~~~, and they also interfere with users who include the dashes in their custom signature.
This has been an issue for some time. See http://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_%28proposals%29&oldid=181428756#Disparity_between_signature_.28.7E.7E.7E.7E.29_toolbar_buttons_and_character_map for an example of Wikipedians annoyed by this issue.
The fix to this is extremely simple. I've created a patch, based on MediaWiki 1.11.0, that works excellently.
attachment EditPage.php ignored as obsolete