I have such a plugin already coded. Works with either mysql or sqlite. Just updated it today to be able to be used as a rank system (so that part is already done), and added natives for other plugins to use it based on minutes played or rank. If using mysql (I know youre not, but others may be), it tracks time both globally, and separates out into servers as well. The structure will give you an idea of what im talking about...
Granted, with mysql, the id and server fields are gone.....so it is just (2) - (7). I do need to make one tweak for sqlite, but that is nothing big. PM me if interested.
Alternately, you might be able to tweak the other plugins suggested here for your needs.