I'm not so curious about that, as I am about whether or not you can get it to either not show the date in your profile, or get it to not update.
To clarify with the circumstances, I have an old name which I used to use quite a lot, was always my name, has full membership, etc. however there's someone I wouldn't want to know I still use that name. Since all the FAQ's say you can't really change your name I was curious if I could set it so that it won't update my last sign on date... but now that i'm thinking about that, if I ever posted, they could still search for my posts and know I was still using the name, even if last sign on wasn't updated, so I suppose it wouldn't help... I guess I answered my own question and will have to continue to use this name and just keep posting until I get privelages back.