BC: prequel
AC: Sequel; 2 years
DC: Sequel to the sequel; 1 year I believe
CC: Uknow

My thought on CC is a prequel and BC is the prequel to that. Why I think this?? Because look at it. CC is Crisis Core. BC is Before Crisis. It can either be that or just a title. Most people will say it's just a title though. But those are my thoughts...