From SC4 Encyclopaedia
A signature is the text which should be added to the end of every comment made on discussion pages, it can be inserted by typing --~~~~.
To use a custom, or non-standard signature, just create a subpage of your user page called sig (must be lowercase) [e.g. User:Freedo50/sig] and type your new signature there. This will then be automatically included instead of the default signature when you type --~~~~. If you create a custom signature, there is no need to include the date/time in it, this is automatically added by the software.