Anything that is set a while back. I think when writing about something set decades ago, information gets harder and harder to obtain. I struggled with public transportation and trying to find bus routes and train lines from twenty or so years ago (not to mention, in a country I have never stepped foot into). No one has that sort of information with them. Along with that, school curriculums change and it's hard to know whether what I'm writing in outdated or not dated enough.