what does the dash mean for last login date?
Using Jamroom
It means they don't have a 'user_last_login' key set on their user info in the database.
The most likely reason for that is that they have never logged in, but it could also be that they have never logged out too so they've always been logged in so its not been set.
Another plausible reason could be the key got deleted. Not sure why or where that would happen but its not completely out of the question.
It should be updated when the session is re-created.